Sand and ballast don’t mix

22nd April 2014

Although ballasted track remains the conventional method of constructing railway lines, the system is susceptible to ballast contamination in desert and wet areas as well as high-wear in heavy use applications. This can lead to high maintenance costs and, sometimes, even track failure.

Since the 1950s, ballast-less track systems have evolved for specialized applications such as high-speed rail and tunnels, but at a significant initial cost premium. What is needed is an affordable solution for areas with poor ground conditions for which conventional ballasted track is not suited.

Ballasted or ballast-less?

Conventional ballasted track consists of cross-lying timber, steel or concrete sleepers on a ballast bed of crushed quarry stone which in turn lies on a wide earthworks formation. Essentially, it is a bridging system whereby sleepers resident on resilient ballast are bridged by rails that are strong (heavy) enough to accommodate the bending stress required by the train’s axle load specification.

Ballasted track is expensive to maintain given that it contaminates easily, requires access to quarries and ballast supply trains and has to be cleaned and renewed using specialised machinery. All of these can present significant challenges to those responsible for railway construction and maintenance in developing countries and in areas of difficult terrain.

Different ballast-less track systems are available and being used around the world, mostly in the category of track slab systems. These include systems with discreet rail support, such as a hand-laid reinforced concrete slab with baseplates; RHEDA2000 – where standard sleepers are cast into a continuous slab; Sonneville Low Vibration Track (LVT) and the Japanese reinforced concrete roadbed system (RCRS) that uses precast concrete slabs.

Ballast-less track systems with continuous rail support include Paved Concrete Track – PACT and Embedded Rail Structure (ERS) where the rails are embedded in an elastic boot, cast into a continuous concrete slab.

Longitudinal support

The technological debate between continuous vs. bridged support of rails has some of its origins in Brunel’s “Baulk Road” system installed in the 1800s on the Great Western Railway in the UK. In these systems, each rail is supported along its entire length by a baulk or longitudinal sleeper. Gauge is maintained by the use of tie-rods between the two baulks.

The T-Track system, privately developed in South Africa since 1989, is a ballast-less track system that competes on an equal footing with conventional ballasted track on initial and major upgrade costs, whilst maintaining the substantial cost advantage of ballast-less track. It is an integrated track system comprising both the track substructure, characterised by narrower layer works (compared to conventional ballasted track) and the track superstructure, characterised by a modular articulated beam-track arrangement.

Track modules

The basis of the T-Track system is the track module. This consists of a pair of reinforced concrete beams which are wet-cast into geotextile bags and separated by gauge bars. Stirrups take conventional rail fasteners (Pandrol, Vossloh, Unit Rail) and continuous pads along the length of both beams provide a resilient base for the rail itself.

The modules sit directly on a prepared formation that is narrower than that required for ballasted track and are grouted into place. The design and stiffness of the modules and the formation is carefully calculated using finite element analysis to obtain the optimum performance and support for the track modules.

There are three basic designs of module. Tubular Modular Track (TMT) is used on straight and curved sections. Modules are precast using the same mould.

Modular Tubular Turnouts (MTT) are precast in sections using a double-sided mould to accommodate left and right-hand turns. Modular Tubular Level Crossings (MTLx) – these are an adaptation of TMT using precast inserts which fill the void between the beams to establish the level crossing.

All three types are available for standard gauge (1,435mm) and narrow gauge (1,067mm and 1,000mm) although other designs are perfectly feasible. MTTS are designed for UIC and voestapline VAE designs for 1:9, 1:12: 1:20 secant and tangential turnout sets, and will soon be availiable for the range of Vossloh turnouts.

System design

The T-Track structure is designed using a conservative 2D model to an ultimate limit state (including fatigue). This is verified against a 3D model concurrently during formation design. The serviceability limit state is set at soil deformation. 3D tie beam design and analysis is verified by hand calculation.

Design considerations include the required axle load and speed, rail size and the underlying soil stiffness.

The rail-track with the formation is modelled, with three dimensional (3D) finite element models (FEM), to determine the stresses and deflections (behaviour) of the track structure. The FEMs are solved with a transient dynamic solver, with the linear elastic material properties with damping coefficients for the system with moving load applications.

The FEM has been calibrated extensively over a number of years for heavy haul operations by the Transnet Freight Rail Track Test Centre in South Africa, as well as the University of Pretoria using measurements from instruments such as pressure plates/ cells at the interface between formation layers and multi-depth deflectrometers (MDDs), typically at three stations, with three holes per station and six MDDs per hole.

In use today

T-Track exhibits a low initial cost and overall lowest total cost of ownership when compared to other track systems. Project cost compares favourably with ballasted track, but has lower maintenance cost, exhibiting a doubling in rail and turnout life and an improvement in weld life. The impact of Tubular Modular Track on project cost is significantly less than slab-track, as it requires a narrower formation as ballasted track. Tubular Modular Track project costs decrease with increasing use over the length of track, as less transitions are required and economies of scale in manufacturing and installation logistics increase.

The ballast-less T-Track system represents a major cost breakthrough for use as a generally affordable rail track solution. It has developed a strong multi-disciplinary value proposition to displace conventional ballasted track and slab track systems in many cases.

And it is not new, unproven technology. 600km are in use in the mining industry while the first surface track was laid for freight traffic in 1990. Since then there have been many successful applications in South Africa, USA and Canada with some installations conveying in excess of 80 million gross tonnes per annum with axle loads of up to 32 tonnes.

Turnouts have been in operation at Ermelo Coal Yard since 2000 without any significant maintenance and have now carried more than 950 million gross tonnes.

Ideal in sandy environments, the first T-Track was installed in Saudi Arabia in 2008. Not only does it remain perfectly resilient, it has maintained its original geometry within 2mm tollerance. Today, having endured some 125 billion gross tonne kilometres in extreme conditions over many years, this technology is now fully proven and certified for operations.

With its lowest cost of ownership and scalability into large projects this technology could well revolutionise the way rail track, especially on freight lines and in difficult terrain, is constructed in future.

Innovation conference gets bigger and better

18th April 2014

With more than 200 delegates from over 90 organisations and 33 display stands, the Railway Industry Association’s sixth annual Technology and Innovation Conference on 18 and 19 March was a much bigger event than last year. Writes David Shirres

Introducing the conference RIA’s technical director Francis How reflected on the past twelve months being eventful for rail innovation with the formation of the Transport Systems Catapult. With some justification he felt that RIA had played a significant role in placing innovation at the forefront of the UK rail industry’s agenda.

Red, green, yellow or blue

Delegates soon found that this year’s conference was to be more people-focused. Jonne Ceserani of Power & Grace led a number of highly interactive sessions for everyone to find out whether they had Red, Green, Yellow or Blue personalities. The serious side to this was that successful innovation needs collaboration from all types of people who need to be open to each other’s ideas. For example your writer had always considered that rail capacity was constrained by signalling systems but was persuaded that train braking also needs to be addressed. Speaking to The Rail Engineer, Francis How advised that RIA felt this to be an important part of the programme as the way people act can be a significant barrier to innovation.

During this session those present were reminded that it was wrong to consider that Charles Darwin’s theory of evolution concerned survival of the fittest. Illustrating the importance of innovation, he actually said: “It is not the strongest of the species that survives, nor the most intelligent that survives. It is the one that is the most adaptable to change.”

Intelligent mobility

Towards Intelligent Mobility is the aim of the Transport Systems Catapult (TSC) as was recently explained in The Rail Engineer (issue 111, January 2014). Its chief executive, Steve Yianni, explained how the TSC is encouraging a collaborative approach to developing integrated transport systems so as to meet the challenges of population growth, congestion and increased energy costs.

Most of the TSC’s current projects will use the improvements in digital connectivity that was the subject of a presentation by Mike Short of Telefonica. As an example of the exponential growth in data transmission, Mike advised that the data transmitted in one year’s operation of Telefonica’s 4G network was equivalent to the previous eight years of 3G operation.

The big increase in the use of smartphones will result in 80% of the UK population having one by the year end. Such phones are currently used for an average of 128 minutes per day.

Those who use such devices are also getting younger, as Mike illustrated by reference to a popular YouTube clip “A magazine is an iPad that does not work” showing a one year old trying to get a magazine to do the things an iPad does. All these factors have huge implications and opportunities for the rail industry.

Innovation in Franchising

Peter Wilkinson, director of rail franchising for the Department for Transport (DfT), believes that rail franchising has been a success. Passenger numbers have doubled since privatisation and 60% more freight is carried. He also felt that few, if any, railways operated so well in such a dense mixed traffic network. Nevertheless, he recognised that rail franchising had to change. For example, a major shift in the DfT’s thinking was that it now recognised that passengers had to come first.

The DfT also now believes that franchising has to foster innovation. As a result, the DfT is making it clear to bidders that quality and innovation is required. This requires new rolling stock as extending the life of the existing stock is not a clever strategy. Peter also felt that the railway was considered to be operating at full capacity only because of self-imposed constraints for which innovation is also required.

He announced that the DfT has created a £50 million Innovation in Franchising fund. This will be available over the next three years for projects covering the East Coast, Trans Pennine and Northern franchises. It will fund innovative projects in these franchises that have a difficult business case or which may not deliver a commercial return during the franchise period.

Sharing experience

David Johnson of DGauge shared his experience of developing an innovative product: a dynamic pantograph gauging technique that allowed restricted clearance tunnels to be electrified without excessive cost. His presentation considered the seven tripcocks of innovation to be: the innovative source; the business case; where does it fit; how does it relate to standards; what approvals are needed; what risk does it import and quadruple the time.

Describing innovation from a rolling stock leasing company’s perspective, Mark Hicks of Angel Trains described the short and long-term tensions between franchise period and operational life of assets. In doing so, he made the distinction between incremental innovation (LED headlamps) and driverless trains and, like many speakers, stressed the need for collaboration.

The pods at Terminal 5

For those using Heathrow, the pods at Terminal 5 offer a glimpse of the future today. Since May 2011, they have carried 950,000 people over two million kilometres of driverless operation at reliability level of over 99%. These pods carry up to four people and are part of a Personal Rapid Transit (PRT) system that provides on-demand transport with an average 10-15 second waiting time direct to the required destination.

Describing the system, David Marron of Ultra Global PRT explained how it could distribute people around busy congested environments at less cost with less disruption. The pods are battery powered and laser guided in a dedicated guideway. A central control manages the driverless system that can have four second headways, enabling it to carry around 2,500 people per hour.

David emphasised that, with this capacity, PRT systems are not intended to compete with light rail. Instead they offer ‘last mile connectivity’ at a cost of around 40% of the typical £20 million per mile for light rail infrastructure.

The chairman’s view

With his extensive experience at Ford Motors in product development and R&D, it is not surprising that Network Rail’s chairman, Richard Parry-Jones, had some fresh insights to offer the conference. His presentation also echoed previous speakers such as the importance of starting with the customer and that there is “no room for work that is not collaborative”.

He felt that Network Rail faced four technical key issues: electrification catch-up; digital asset management; automatic traffic control and the signalling revolution for which it needed to learn lessons from other industries. For example, there are “many factory concepts that belong on the railway.” The industry also needed to imagine a future with free and infinite digital processing and transmission.

He could understand why railway engineers were nervous about the impact of new technology but felt this risk could be controlled with a robust technology introduction process. On the subject of workforce safety, he considered that this had to be improved and that there were safety solutions in the oil, gas and mining industries that Network Rail should adopt.

Looking back, moving forward

David Clarke of the FutureRailway team announced that there would soon be one less acronym to contend with. EIT is to be dropped as his Engineering Innovation Team merges into FutureRail, a programme with a virtual team from Network Rail and RSSB which is working with industry to deliver the Rail Technical Strategy. It has direct funding of £125 million in CP5 which, with industrial match funding, gives £250 million.

The UK supply chain, which generates business worth £7 billion per annum, only exports 10% compared with 60% and 70% for the automotive and aeronautical sectors. FutureRailway aims to get the supply chain to meet the industry’s demand for innovation and as a result also increase opportunities for export into a global rail market worth £100 billion in 2010 and growing at 2.7% per annum.

To date, FutureRail had provided innovation funding of £30 million, matched by an equal sum from industry. This had been used for various competitions attracting a total of 300 entries, 75 days of testing at specialist facilities and demonstrators such as the independently powered EMU. In addition, support had been provided for DfT’s innovation in franchising.

Currently FutureRail has five live competitions, details of which can be found on its website. David also encouraged delegates to support the Young Railway Professionals’ event Next Generation Rail on 11-13 June.

The conference award

As at previous conferences, a key feature was the RIA/ RSSB Innovation Award for which there were four finalists. Brecknell Willis presented their active pantograph proposal which is intended to reduce service disruption, allow high speeds with existing overhead equipment and provide real time monitoring of OLE.

FLE structures described its idea for an OLE cantilever arm made from non-conductive material, thus eliminating the need for an insulator.

A previous winner, Park Signalling, introduced a proposal for remote condition monitoring (RCM) of signalling equipment not enabled for RCM – which is most of that installed on the network. This involved transmitting LED indications on printed circuit boards to a remote technician’s terminal, eliminating the need to visit the location concerned in order to diagnose faults.

Reliable Data Systems presented its proposal for a video train positioning system which analysed a video of track in front of the train to determine distance travelled. It can also determine the train’s location which can then be displayed on a track map.

Anson Jack of RSSB was glad to note that the delegate’s vote matched the view of the judges which was that this year’s RIA/RSSB Innovation Award, worth £300,000, should go to Brecknell Willis. As a result, the winner expects to produce a prototype of its active pantograph in 24 months’ time.

A further prize was the Mystery Shopper award for the best stand. This went to Thales for their display featuring advanced technologies yet to be used in the rail environment. These included precise positioning information which provides the exact location of individuals in hazardous environments. This works through walls and obstructions and its ‘Through-Wall’ radar technology can even detect the gentle breathing of someone sleeping in a room!

Innovate or die

As was made clear at this event, innovation is the only way that the railway can meet its 21st century challenges. So it is good to learn of worthwhile initiatives at the annual RIA conference which is now a ‘must attend’ event for anyone with an interest in this field. Through its conferences and unlocking innovation scheme, RIA is fulfilling a vital role to promote rail innovation.

Yet there remains much to be done. Informal discussions with suppliers revealed that commercial pressures remain a significant barrier to innovation. However, the oft-used word ‘collaboration’ indicates that this problem is recognised. A number of speakers challenged the premise that the railway was full and felt that those in the industry need to come up with radical solutions to the capacity problem.

The conference was certainly an education. Charles Darwin’s comment in his Origin of the Species can be paraphrased as ‘innovate or die’. With the competition now testing road trains of convoyed cars and driverless cars, this is a clear message for the rail industry.

For more information, check out:

www.shift2rail.org

www.futurerail.org

www.rruka.org.uk

Can you teach an old dog new tricks?

17th April 2014

A lot has been written about sustainability from a corporate point of view. However, as a mature individual, and one that has held senior positions for a number of years in a variety of industries, sustainability in the environment is a new concept and one that, at the beginning, I struggled with. Writes Barry Dilks

Having delivered a number of substantial projects, I am well aware of environmental issues. I have experienced delays and unplanned expenditure protecting wild orchids, great crested newts and slow worms. I have had to engage a would-be Swampy – camped outside a badger set to establish if it was in use. I even built a 120-room doormouse hotel that ran for 30 metres parallel to the railway so we could assess the potential impact on wildlife. Sadly, the facility was closed down when not a single booking materialised after three months.

These were all necessary steps in today’s environmental culture as a combined commitment to protect our wild life and the environment. But sustainability …. what on earth is this all about?

Corporate policy

I decided to investigate the UK Power Networks Services (UKPN Services) corporate position and, not surprisingly, discovered that we have processes, procedures and guidelines supporting this initiative. We recognise Responsible Procurement, Protection of the Environment, Waste Management, Pollution Control and Conservation, but it was clear that using the word ‘Sustainability’ meant more than that.

So I engaged with the Network Rail Thameslink Programme Environmental team who were in the process of drafting an Environmental and Sustainability Statement with a vision of linking it to a number of Key Performance Indicators (KPIs).

I left that initial meeting with a headache and, I will be the first to admit, a concern that I still did not fully understand what was clearly a hot topic.

It was time for some new blood and a young mind set which could establish a sensible approach and provide clarity and guidance to the delivery team and, just as importantly, me! We recruited a young environmentalist, Maria Siakovelli, and she was given the task of understanding Network Rail’s expectations, map those against our corporate undertakings and provide a gap analysis.

The good news was that the gap analysis demonstrated that our corporate view and Network Rail’s expectations were in general aligned. Phew, that was a stroke of luck – or was it?

Reporting and monitoring

Having established that we (UKPN Services and Network Rail) are aligned in strategy according to Network Rail, the rail industry and government expectations, what do we do next? Like many other companies, we are pretty poor at reporting all the good things that we do and how we do it, collating data and then ascertaining whether we can be more effective through the analysis of such reporting.

Working with the Thameslink Programme and Network Rail’s environmental and sustainability team, Maria established a number of KPIs that would be the subject of continuous monitoring and reported as a dashboard with statistics and trends. Being young, with lots of passion and bags of energy, Maria took a view that we could do more with minimal investment but it would require effort and buy-in from the project and construction teams.

It was Maria’s passion that made me sit up and listen. Not being a natural tree hugger, and having been busy delivering construction projects all my life, I sceptically viewed the concept as a risk to our delivery schedule.

I was initially quite vocal regarding the ‘supposed’ benefits. What difference can we make? We do not execute huge civil-type programmes – we are a highly competent electrical contractor, working generally on small footprints, so how on earth could we make a difference?

Leading by example

What Maria told me was: “A small contribution can make a huge collective difference.” She sat me down, debated the issue, challenged my views, brought me out of my box and took me for a walk in the potentially greener world. Maria convinced me that we needed to encourage everyone to think sustainability within the environment so they had a better understanding. We had to lead by example and tell everyone what we were doing as a company, why we were doing it and the benefits our contributions could make to the greater scheme.

Hmmmm… Knowing that typical construction crews simply want to get the job done to a high degree of quality (in a safe manner of course), I remained a little sceptical. So Maria produced an initial dashboard of what we were already doing as best practice in project management terms. She cited the usual activities such as segregating waste, recycling waste rather than sending it to landfill, utilising temporary electrical connections for welfare units rather than diesel powered generators, managing delivery logistics better, and the like.

The initial report provided statistics of actual compared against ‘what if’, and the results were quite staggering. She modelled the results against the overall number of sites we were scheduled to deliver and demonstrated we could make a difference to our environmental impact if we compiled statistics across several projects and the company overall.

OK, I was convinced. I still wasn’t exactly what you would call a tree hugger, but I have to say the statistics certainly got my attention.

Challenging Maria

But I threw Maria a curve ball and challenged her again by stating that, in everything we do, we must always ensure that expenditure and effort bring short and long term benefits. I asked her to tell me what we could do, what investment was required and how quickly would that benefit be realised.

Maria initially concentrated on ‘what can we do today that can bring immediate benefits with little or no financial investment other than time and effort?’ She set out a strategy of educate, plan, encourage feedback from the bottom up, capture data, educate some more, plan again (think outside the norm), report back on what we have done and then educate everyone again on the impact and benefits provided.

And so Maria started the education process with the project team and site crews. She stressed the benefits of what we were doing using the data from historical works and overlaying future work sites. The feedback was refreshing. Everyone thought it was a good thing to do and even provided suggestions to consider some other savings such as – why do we have empty vehicles? What about ECO cabins? Why do we scrap so much?

When we start on a new site, we initially level the construction area, remove the waste and then fill with appropriate foundations – say type 1 for construction. We looked at this process for one site, namely Ifield in West Sussex. Typically we would arrange for a tipper with a grab facility that would set off from Godstone empty, pick up the site spoil and return to base. That spoil would then be segregated, the recyclable material sent to Gatwick recycling and the remainder despatched to Smallfield landfill centre. Once the site was ready, the same company would deliver type 1 to site and return to base empty.

St Pancras – Sheffield Line Speed Improvement Project

14th April 2014

Many people, including the author, have long argued that the East Midlands has suffered for too long with a second rate main rail line. In fact, that has been the case ever since the demise of the majority of the Great Central line at the time of Dr Beeching. Writes Chris Parker

The Great Central was the only British main line built to continental gauge, and was constructed from the outset as a main line railway. Its alignment was designed for what were, for the time, high speeds. There were separate tracks on much of the route to keep freight and stopping traffic out of the way of express trains.

Making the best of it

For whatever reason, Beeching decided to destroy most of this route, retaining instead the twisty, tight Midland Railway main line (MML). Why was this the chosen option? It can’t have been about making the railways more efficient and effective.

Whatever the reasons, the option to reinstate the Great Central line is now confined to the ‘too difficult’ category. Too much of the alignment has been sold and built upon or ploughed up for any realistic chance of a revival. The East Midlands therefore has to make the best of what it has, at least until the arrival of HS2.

That means making the best of the MML. Network Rail and the other stakeholders in the line, particularly East Midlands Trains, have been collaborating to try to make the line rather more of a silk purse than a sow’s ear. A plan was developed as part of Network Rail’s Control Period 4 (CP4) commitment and The Rail Engineer recently visited the East Midlands Control Centre in Derby to meet scheme sponsor Kevin Newman and look at the achievements and challenges of the project.

The £70 million scheme is intended to cut eight minutes in either direction from the journey time for Class 222 Meridian sets running between Sheffield and London St Pancras. Five minutes of this target is being delivered through infrastructure works along the route. The remaining three are coming from reductions in the timetabled engineering allowances on the route, which are being made available by other transformational programmes on the part of Network Rail.

Planning and collaboration

The project has used the so called ‘third way’ approach to line speed improvement that was the basis of much of the similar work done on the West Coast main line. This involves detailed examination of the theoretical and actual track geometry on each line to determine the most cost effective practicable increase in speed attainable on each section of line (if any). The ruling speed of the line was already 125 mph, but relatively little track was actually running at this speed before the project commenced.

Planning and execution of the works depended heavily upon collaborative efforts between Network Rail, East Midlands Trains and other stakeholders.

For example, there would be no point in increasing the permissible speed at any given place to a speed higher than that practically achievable by the trains. Depending on the circumstances, this might be constrained by the accelerative capabilities of the sets, or by their braking characteristics. There would be no point in a line speed that trainscouldn’t reach by accelerating away from a regular station stop or permanent speed restriction, nor would it be useful to extend a high speed beyond the point at which trains would always need to start braking for a similar stop or restriction.

Implementation has been made easier by collaboration between Network Rail and the train operators. East Midlands Trains, in particular, has been collaborating closely with Network Rail over the various works around Nottingham, and this involvement and assistance has been equally vital in the MML project.

Four phase commissioning

The original plan was to substantially complete the whole of the project in time for the last national timetable change on 8 December 2013, commissioning the new works in four phases. The first of these, Derby to Chesterfield, was completed on time on 16 November 2013. Elstree to Sharnbrook was due to complete, and did so a week later, on 23 November. East Midlands Parkway to Derby also finished on time on 30 November 2013.

The missing section was East Midlands Parkway/Sharnbrook, which was originally due to complete on 7 December 2013. However, it was foreseen that there was going to be an issue with track condition between Kettering and Loughborough. Such were the track conditions over this stretch that it was decided that it would be impossible in practice to drive to the increased line speed in between the required condition-of-track speed restrictions.

The decision was therefore made to defer the completion of the affected section until March 2014, to permit the necessary track renewals to be completed. This was a joint decision as Network Rail had flagged up the situation early and discussions with the train operator determined the way forward. Network Rail held a series of roadshows for train drivers and other staff to explain to them why the situation arose, what was being done about it and the timescales involved.

Due to this delay to the one section of the project’s works, East Midlands Trains’ performance under the PPI measure suffered some days. With lower speeds than planned on the affected route section, it was difficult to recover from other problems on the route. There was also some impact on the train operator’s ‘right time’ performance measure.

Despite that, East Midlands Trains continued to work co-operatively with Network Rail, for example by assisting in finding the infrastructure operator the necessary access opportunities for the work. Though they would ideally have liked an instant solution to the situation, they understood that this was not practical, and so they helped expedite matters where they could.

Aside from the condition-of-track renewals required between Kettering and Loughborough, there were some S&C renewals (including Flitwick for example) still not complete at the time of the introduction of the new timetable. A number of track realignments could not proceed until completion of planned bridge reconstructions in preparation for the forthcoming electrification of the route.

Whilst there were significant timetable improvements, the full benefits of the project were not input into the December 2013 timetable change, but will enable further enhancements in the future. In the interim, it is heartening to see this further evidence of mature, collaborative working between Network Rail and East Midlands Trains on another major project.

Recognition must also be given to the other players in the project. Carillion, Network Rail’s major framework contractor, undertook signalling and some track works alongside Network Rail Projects. AMCO was the bridgeworks contractor, while Atkins and Babcock were respectively responsible for the signalling and track designs. Signal testing was by TICS.