HomeHeritageBuilding the P2

Steam locomotives are inefficient and costly to operate, hence British Railways replaced them with diesel and electric traction nearly fifty years ago. Today, however, steam-hauled specials are still very popular – and profitable. There is no denying the enthusiasm for steam trains on which people are willing to spend the time and money needed to keep heritage lines and their locomotives operational.

Britain has over thirty major heritage lines which operate some of the 180 steam locomotives that were rescued from the scrapyard after BR ran its last steam train in 1968. 22 of these old locomotives are authorised to operate on Network Rail’s infrastructure along with one new one, the A1 class 60163 ‘Tornado’. Built by the A1 Steam Locomotive Trust, it hauled its inaugural train in January 2009 after taking fourteen years to build.

Tornado is the fiftieth A1 class loco – the other 49 were built in 1948 and 1949 to a design by Arthur Peppercorn, the last chief mechanical engineer of the London and North Eastern Railway (LNER). It has now run over 83,000 miles and is typically used on 20 to 25 main line steam charter trains each year.

Why a P2?

Inspired by the success of Tornado, the trust decided to build another LNER locomotive – a P2 class designed by Sir Nigel Gresley. These were Britain’s most powerful passenger steam locomotives with a rare 2-8-2 ‘Mikado’ wheel arrangement. Only six were built and none survive. Building the P2 was a popular choice as, in a survey undertaken for the trust, 60% voted for it.

The first P2, No 2001 ‘Cock o’ the North’, was built in 1934 and incorporated various novel features including rotary poppet valve gear and distinctive streamlining. All six P2s were different. No 2002 had Walschaerts valve gear and others were fitted with the same streamlined front end as the A4 Pacifics.

The intention to develop the design further was frustrated by the war during which Gresley’s successor, Edward Thompson, had the locos rebuilt with a 4-6-2 wheel arrangement. This was not a success as they did not have enough driving wheels for their power output.


LNER. 2-8-2 P2 Class Gresley [online]
Photo: Railphotolibrary.com

David Elliott, the Trust’s director of engineering, explained that the P2 needed its eight driving wheels to haul fifteen coach trains on the steeply graded Edinburgh to Aberdeen route. He considers it a spectacular design with an almost limitless ability to haul trains. With such power, there will be increased revenue from longer trains, or, as David put it, the further the back coach is from the locomotive, the more profitable the train.

The new P2 will be No 2007 ‘Prince of Wales’. It will be modelled on No 2001, which had Lentz poppet valve gear and a unique front end that cannot be confused with any other locomotive.

David acknowledged that the P2 had its problems. It did not go round curves well and had occasional crank axle failures. Alarming though this might sound, David was certain these problems could be solved by current railway engineering techniques.

3D design

2007 will not be a slavish copy of 2001. Its design will be that of a P2, but altered to resolve problems, improve maintenance, reduce life-cycle costs and meet current and foreseeable operating requirements and standards. Much of it will incorporate proven Tornado components.

The starting point was the National Railway Museum’s P2 drawings, which were copied on a full-size digital scanner. With the advances in computer aided design (CAD), the entire locomotive is modelled in 3D to the rails. Once the frame had been heated to a dull red heat by two large oxy-propane torches, it was nudged by a fork lift truck to the correct shape as confirmed by a laser- profiled template. The rear frame is to Tornado’s design which has the same principal dimensions as the P2 class but is easier to manufacture.

Frame castings were produced by William Cook Cast Products of Sheffield. Existing patterns were used where frame components were common to Tornado, otherwise new while the coupled wheels required a new one.

After casting six wheels, the pattern was given larger bosses and used for the two driving wheels. Unlike Tornado, the cast coupled wheels have no balance weights as metal removal for wheel balancing is expensive. Instead, fabricated balance weights containing lead antimony alloy will be fitted.

It is hoped that the amount of alloy for the correct balance can be theoretically using ‘Solidworks’ software. This will sort out clashes before cutting metal, particularly useful for electrics and plumbing. It incorporates a finite element analysis package and also produces really nice pictures for advertising.

Frame assembly 2007 underside 28-1-16s

Design work started in 2013. The plan is, subject to funding, to complete the locomotive by 2020 and, after trials and testing, have it operating
in 2021.

Flat pack start

Construction started in April 2014 with the rolling and flame profiling of the frame plates at Tata’s Scunthorpe steelworks. Frame holes were drilled and the plates machined all the way around, using CNC machines, by Boro’ Foundry at Stourbridge. This reduces the risk of cracks and provides accurate reference points.

Construction started when a flat pack frame kit arrived at the Trust’s Darlington workshop where the frames were set up on stands and held together with threaded rods prior to fitting their stretchers. Frame assembly will require 1,065 driven bolts with an interference fit.

The 30mm rear frame plates were bent using a novel technique. A bending former was positioned on the inside of the bend and tack welded polystyrene patterns were made. These were CNC machined from the 3D CAD drawings by Bakers Patterns of Telford. Axle boxes were then machined by Timpsons Engineering of Kettering, which also manufactured the roller bearing spacers and rings.

Preventing axle failures

Wheels were another early programme item. These have now been cast and proof machined. Carrying wheels (four for the engine, eight for the tender) were cast from existing Tornado wooden patterns determined using 3D CAD without using a balancing machine and that this will satisfy the vehicle acceptance body.

The 6’ 2” diameter tyres had to come from South Africa, the only place that can manufacture tyres of this size. Roller bearings will be the same as on Tornado, a degree of interchangeability that has already proved useful – during Tornado’s recent overhaul, bearings intended for the P2 were ‘borrowed’.

Having the wheels fall off was a problem for the original P2s. The six locomotives had five crank axle failures within ten years. Fortunately, there were no derailments. The crank axles broke at low speed as the train started, when they were subject to maximum torque generated by piston forces of up to 34 tonnes. Furthermore, the eight-coupled wheels rarely slipped, which would allow excess torque to dissipate, so all of the tractive effort was forced to pass through the cranks. The crank pin keyway design also made the pin susceptible to crack propagation.

To ensure 2007 suffers no such failure, Mott MacDonald undertook a finite element analysis of the crank axle. This showed that an improved keyway design would significantly reduce the probability of failure but not completely eliminate it. Hence the axle diameter is to be increased from 95/8 to 10 inches to take it further out of the fatigue regime. With improvements in bearing technology, this increase in diameter can be accommodated within the P2 bearing housing.

Patterns [online]

Poor curving

When introduced into service, the P2s suffered from indifferent curving ability. They derailed on poor track in yards and could exert high forces on curved main line. Gresley’s pony truck swing link suspension tended to lift the front of the locomotive and off load the leading coupled wheels. High track forces arose from the swing link suspension’s uncertain centering force on poor track.

The LNER found a solution during World War Two when its Doncaster works built LMS class 8F locomotives which had a pony truck side control system that used springs to exert a side force which doesn’t lift up the loco.

The Trust appointed DeltaRail to model the interaction between the P2 and the track using Vampire software. This confirmed the original design’s problems and has been used to refine a pony truck design with side spring control and to determine coupled wheelset clearances. As a result, 2007’s track forces will not be higher than Tornado and so will be able to run on the same routes.

David explains that this was not straightforward. Vampire, intended for modern bogie vehicles, had to be modified for a vehicle with a rigid chassis and for a steam locomotive’s unbalanced reciprocating forces. Extensive data collection during Tornado’s main line testing was used to set up Vampire to accurately model a steam locomotive.

Cylinders and valves

With 21” cylinders, the P2 design is two inches wider than Tornado, and so would be out of gauge where track has been moved to reduce the platform gap. The cylinder diameter for Prince of Wales has been reduced to 193⁄4”, and a further 3⁄4” has been saved by using a fabricated steel rather than cast iron cylinder block. To retain the same power output as other P2s, 2007’s boiler will operate at 250psi instead of 220psi.

Gresley believed in the potential advantages of poppet valves. Hence, unlike other railway companies, LNER trialled Lentz poppet valve gear on various locomotives, including P2 2001. To assess its effectiveness, 2001 was sent to the Vitry-sur-Seine locomotive test plant in Paris to be compared with classmate 2002 which was fitted with Walschaerts valve gear. The results did not favour the Lentz arrangement, partly due to the materials technology of the time.

For the greatest efficiency, a steam locomotive needs to operate at maximum pressure and control power by ‘cut off’, the proportion of cylinder travel during which steam is admitted. This needs to be finely adjusted according to power demand. As built, the Lentz valve gear had infinitely variable cams. However, these became worn by their roller followers. The solution was a series of stepped cams making it no longer possible to fine tune the cut off and keep the regulator fully open for maximum efficiency.

With improvements in the heat treatment of steels and modern tools, a durable infinitely variable cam can now be produced. When 2001 was built, cams were turned to their approximate shape, heat treated, then ground down to precise dimensions, removing some of the hardened surface. Now CNC machines can turn cams to their final shape so no further metal is removed after heat treatment.

Darlington workshop [online]

A problem fitting Lentz valve gear to 2007 was the lack of drawings in the National Railway Museum. However, research undertaken by Andy Hardy, the Trust’s P2 archivist, found an extensive set of drawings of the Lentz valve gear fitted to South African locomotives.

Further drawings and manuals of a modified Lentz gear produced by the American Franklin Company have also been obtained. David explains, that by chance, he made contact with Charles Smith, whose father, Vernon, was a senior engineer with Franklin. As a result, the Trust was able to scan the Lentz valve gear information that Charles held.

Boiler and smokebox

2007 is to have Tornado’s type of boiler. This was manufactured by Deutsche Bahn Meiningen steam locomotive works in the former East Germany where main-line steam operation continued until the mid-1980s. As this boiler is 17 inches shorter than the P2 boiler, 2007’s smokebox will be 17 inches longer. In practice, it was found that this extra boiler length did not raise extra steam as firebox gases cooled at its far end.

An advantage of the longer smokebox is a larger vacuum reservoir to smooth exhaust pulses and stop the fire being pulled apart. A particular challenge was the flat-topped smokebox door required by the streamlining. This could not be made from a spun tank end like Tornado. Instead the South Devon Railway’s boiler press was used to form heated steel plate into the required shape using tooling made

from the 3D CAD model. The smokebox door frame is to be machined from 90 mm thick boiler plate.

Tornado’s pattern could not be used to cast the double chimney due to the streamlining’s sloping smokebox. Hence a new pattern was made at a cost of £6,500. However, the 3D CAD model confirmed that streamlining would not prevent Tornado’s complex superheater header being fitted in 2007’s smokebox, thus saving £18,000-worth of pattern making.

2001 Paris

Electric oil lights

2007’s headlamps each have seven LEDs, equivalent to a 150-watt halogen bulb. These satisfy Group Standard requirements and fit inside a replica of the original P2’s oil lamp.

Its electrical system will be similar to Tornado’s, which has three miles of low-smoke zero-halogen wiring from 230 separate wire runs. It has a steam turbo generator (32 amps at 28 volts) and, on its tender, a belt driven alternator (200 amps at 24 volts) and two 24 volt 65-amp hour battery banks. The wiring will run in ‘hammer proof’ 6mm stainless steel trunking with removable covers for easy wiring alterations.

Tornado has two separate electrical systems, essential (for TPWS, headlamps) and auxiliary (lighting, drain valves). The essential supply’s control panel is under the driver’s seat, whilst the fireman’s seat is over the auxiliary panel. Strip lights illuminate the motion for cleaning and maintenance – some drivers even use these to make the locomotive look good at night!

The need to fit ERTMS signalling in the future has been considered. The power supply is rated for its power demand and there will be an ERTMS cubicle in the tender.

Fulfilling the promise

Building the new P2 will cost an estimated £5 million. Raising this sum is a challenging target, but not unachievable as £3 million was raised to build Tornado. The Trust has various innovative fundraising schemes such as its Founder Club, which alone raised £450,000, sufficient to get the project to the point of manufacturing the frames.

There is no doubt that the company has the skills and experience to tackle the complex mechanical engineering project of building the new P2. Whether it can do so by its planned 2021 completion date depends on the rate at which funds can be raised.

The P2 was a promising design that was never developed to meet its potential. The use of modern technology to remove known flaws will fulfil this promise by producing one of Britain’s most efficient and powerful steam locomotives. It is interesting to speculate what Sir Nigel Gresley could have achieved had he had the technology available to the A1 Steam Locomotive Trust.

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