The Asfordby (or Melton) test track has had an interesting history. It originally formed a part of an important rail route between the East Midlands and London. Writes Chris Parker
For a time in the 1970s, it was the route that the Nottingham Pullman services used to get to the capital after the closure of the Great Central Railway. Then the Nottingham/Melton Mowbray section of line was closed to traffic itself, and Nottingham services to London had no alternative but the Midland route via Trent Junction.
This was in many ways the start of the interesting times for the newly closed line, however. Having been taken into use as one of British Rail’s test tracks, it was used by BR Research for many things, including trials of the ill-fated Advanced Passenger Train (APT).
Ironically, this project, which really failed through loss of nerve on the part of the politicians, established many details of science and engineering about tilting train technology. That, in turn, led to the Italian Pendolino trains that now run on the West Coast main line in the UK, with counterparts in many other countries. The real irony lies in the fact that those very Pendolinos were themselves tested on this same track before entering UK service.
The test track has been used most recently by Serco, which leased it a few years ago so that they could fulfil their contract with LUL for the testing of new rolling stock for the London Underground. When The Rail Engineer visited recently, there were still trains in LUL livery on site, a strange sight in the middle of the East Midlands countryside.
Parts of the track were electrified on the 25kV overhead system for the Pendolino tests, and similarly parts now have third-rail DC electrification for the LUL tests.
Ballasted vs ballastless
One recent development in the Asfordby tunnel on the test track might have far reaching consequences, greater than either the Pendolino or the LUL trains.
Ballastless track has been a holy grail for forward-looking rail infrastructure engineers and managers for a long time. In the UK, the snag has always been the cost of converting existing lines from ballasted to ballastless.
There seems to be no reasonable debate about the case that the whole-life costs of ballastless track should be significantly lower than equivalent, conventional, ballasted track. The initial costs are higher, though not necessarily as much as one might think, but the costs during the lifetime once installed are so much less that the higher first costs are more than recovered over the next 60 years in service.
The problem, even in BR days when there were no track access charges and the like to be paid by the infrastructure operator to the train operator, no-one was able to show how the costs and disruption involved in removing ballasted track and replacing it with ballastless could be made to appear economically reasonable.
Today, with all the other work that Network Rail has to carry out within its budget, converting the existing railway wholesale looks a hopeless proposition. Yet it is a viable possibility for relatively short lengths of track where there are particular challenges to manage – tight tunnel clearances for example – but not large-scale conversion of line of route railway. Even the great Japanese railways have not managed
to justify conversion of the first, ballasted Shinkansen line to ballastless track.
Trialling three systems
The trials that Network Rail has undertaken in Asfordby Tunnel may offer some hope for a change in this view. A recent site visit was hosted by Network Rail’s project manager Manraj Bhandal along with several colleagues from Rhomberg Sersa Rail Group, the company that provided and installed the trial track systems. Carl Garrud, Managing Director and Chris Kearns, Project Manager were very proud of what their company had to offer, and certainly the quality of the installation in the tunnel by the Austrian project team from Rhomberg Bahntechnik looked very impressive.
The 470 metre long trial consists of three separate installations, a trial length of each of two ballastless track forms and a transition structure designed to ensure a smooth shift between ballasted and ballastless track.
The first section in the tunnel has been re-laid with IVES track. A Rhomberg Rail development, IVES simply stands for Intelligent, Versatile, Effective and Solid. It is not strictly slab track, since it consists of individual prestressed concrete units with rail support assemblies for each rail, separated by a small gap.
The system is ballastless though, the units being laid on asphalt paving, and it does behave like a slab since the 250mm deep concrete units are heavy (one tonne each) and a stainless steel dowel pin is used to restrain the units from moving relative to the tarmac. Not every unit needs to be dowelled and on this site, where the curves are relatively flat, one dowel every fifth unit was the requirement. Where sharper curves, faster speeds or heavier loads dictate, dowels would be installed more often by design.
The installation involved removing the old track then excavating down through the ballast and formation to the required depth. This was done by Babcock Rail using laser- guided machines to give a correctly-canted formation ready for the asphalt. Onto this went 100mm of Type 1 fill, following which a paving machine laid the 250mm of asphalt in two passes.
The IVES modules were then installed on the asphalt and roughly aligned. Rail alignment, horizontal and vertical, was fine tuned during the final positioning of the rail support assemblies using a system of alignment called RhoTAS. This holds the rails and support assemblies in the correct position whilst the latter are grouted into position.
The support assemblies used in the trial were Vossloh DFF units, and the screws fastening them down needed to be tightened to the correct torque after the grout had set. On this site, the grout used needed 10 hours to reach adequate strength, though faster-setting grouts are available.
The IVES system needs no significant wet concrete work as the asphalt is laid to a sufficiently accurate standard (vertically, +0 to -35mm, horizontally to +/-15mm) that the concrete modules sit directly upon it. They are aligned in this way sufficiently well that the final fine-tuning of the rail height and line is easily within the adjustment capabilities of the rail support assemblies.
A company-designed track measuring trolley is used to make these fine adjustments. Two people go through with the trolley adjusting the rails in a two-stage process. On the first run they work to a 2mm tolerance, then to the final design tolerance on the second.
The entire IVES installation is undertaken by a team of eight, including their supervisor. They lay and align the modules, install the rails and align those, and finally grout up and tighten down the rail support assemblies. On completion, the measuring trolley is used to record the final geometry of the track for an as-built record for the client.
The IVES system is very flexible, coping easily with curves, gradients and transitions in much the same way as traditional sleepers. Variants are available for use in S&C, and it is possible to supply units for use as longitudinal rail bearers where that is required. The gaps between the units allow drainage to flow down onto the asphalt and thence away into the subgrade or the track drainage as applicable. Where it is desired, the gaps between modules may be infilled with foam inserts which keep out debris whilst still allowing water to drain. The modules may also be driven upon by road vehicles, a distinct benefit in tunnels where it may provide a way for emergency access or maintenance vehicles.
The second track system under trial is the PORR system, jointly developed by Austrian Railways (ÖBB) and Allgemeine Baugesellschaft A. Porr AG. Like the IVES units, this system relies on an accurately machine laid asphalt base on a 100mm Type 1 sub-base. However, the concrete modules of this system are significantly different. Each slab is five metres in length and weighs about five tonnes. They are only 150mm thick, however, and so they are slightly flexible.
Five jacking screws are incorporated into each unit, one in the centre and one towards each of the four corners. These are used to level the units approximately 80mm above the asphalt base. The flexibility of the modules allows them to twist and bend to accommodate some of the variation in rail alignment that may be required in track transitions, for example. Greater variations in alignment are dealt with by designing and casting modules with the necessary curvature and cross level variation built into them.
In each module there are two large windows, one towards each end. Once the module has been aligned and levelled correctly, self-levelling concrete is poured in through these windows to fill the void between the unit and the supporting asphalt. At Asfordby, the rails are supported and fastened in Vossloh assemblies, as with the IVES modules. However, plans are in hand for production of units using Pandrol Vipa housings as an alternative.
In all, the trial at Asfordby consists of 374 IVES modules and 24 PORR slabs. These make up a total of 470 metres of track, with the PORR section roughly in the middle of two lengths of IVES modules.
The final trial consists of an eight metre long V-TRAS transition module (Vertical Transition Sytem) at the south tunnel portal. This is designed to avoid problems caused by a sudden change in track stiffness where ballasted and ballastless track meet. However, there may also be applications elsewhere, as for example where track problems have arisen at the transition between an embankment and an underbridge.
The module is essentially a steel ladder structure consisting of two steel beams with transverse support plates between them at intervals corresponding to the sleeper spacing. These carry resilient pads for the sleepers to sit upon. The plates have upturned ends to restrain the sleepers from lateral movement.
The installation at Asfordby has a cast in-situ concrete support block under the track at the start of the slab track. The one end of the V-TRAS module is supported on the outer end of this block whilst the remainder sits on the bottom ballast, under the sleepers it is to support. The track is packed or tamped through the V-TRAS unit in the normal way, but the stiffness of the ladder of steel ensures a gradual increase in the overall track stiffness through the length of the transition assembly.
Work is in hand to develop a pre-cast support block for the V-TRAS. This would still need to be placed on a smaller in-situ foundation slab, but the quantity of wet concrete work required would be reduced significantly with this option.
The hope now is that the speed and ease of installation of these track forms may considerably reduce not just the cost of the physical works, but also that of the necessary track access. The use of laser-guided machines to excavate the formation and to lay the sub-base and asphalt base has the potential to make this part of the work a rapid and efficient process. The minimal use of wet trades, particularly with the IVES system, also potentially contributes to speed and efficiency. Finally, the ability to drive rubber tyred vehicles over the completed slabs may have a further advantage, especially where site access is restricted.
Network Rail and its suppliers will now set out determinedly to see how they can make best use of the potential demonstrated here. The fact that the company has recently authorised a methodology for whole life costing and is in the process of integrating this into the company project management process gives grounds for real optimism that ballastless track may be something that we begin to see much more of in the future.
If that occurs, this project at Asfordby will not only have succeeded in its aims but have gone well beyond them. Its original remit was about sourcing innovative means of relaying track where there are gauge clearance issues. It should certainly meet that target as there is significant opportunity to lower the rail levels by using either the IVES or the PORR system. Should it also turn out to offer a route to cost effective track conversion, that would be a fantastic bonus for UK rail.