The classic children’s song The Runaway Train was written in 1925 by Robert E. Massey, Carson Robison and Harry Warren. You may not know this first verse, but no doubt you can sing the chorus of “The runaway train came down the track and she blew…..”

It is a humorous little song. But when the runaway train is a 28 tonne excavator, running down a gradient in the middle of a worksite, with the driver’s foot hard on the brake pedal, and it is still doing ten miles an hour, then it is not so amusing.

And to some extent, that is what has happened on occasion. Road-rail plant is usually a conversion of a machine normally used on roads and rough ground, and if that conversion isn’t perfect then trouble can occur.

Type 9b

Following a detailed risk assessment, which included a number of calculations based on observations and incident reports, Network Rail derived that the biggest risk seemed to be with Type 9b excavators. For readers not conversant with the terminology, there are three types of road-rail excavators, depending in the type of rail wheels fitted.

The reason for these seemingly strange notations is to allow harmonization with Europe; RRVs used within a possession are known collectively as Type 9 vehicles.

Type 9a vehicles have hydraulically operated rail wheels which, when lowered, lift the whole machine clear of the track. The flanged rail wheels are hydrostatically powered and the whole traction and braking system is independent of the original road-vehicle.

Type 9b,also lift the road wheels clear of the track so that the machine only runs on its flanged steel wheels. However, drive and braking still comes through the rubber tyres, which either rub against the steel tyre of the flanged wheel, or on a knurled stub axle which protrudes from it.

Type 9c have flanged guide rails which are let down at each end of the machine to keep it on the track. However, the rubber road wheels sit on the top of the rails and provide traction and braking directly between the tyre and the rail surface.

The problem seemed to be with those type 9bs which were arranged so that the rubber road tyres drive, and brake, the machine by friction against the steel rail wheel tyre. In dry conditions, and on level track, there is no problem at all. The driver puts his foot on the brake, the rubber wheels slow down and stop, and so do the rail wheels.

However, and it’s a big however, occasionally that doesn’t happen. If the interface between the rubber and steel wheels becomes contaminated by oil, or leaf mould, and that then becomes wet, and the rubber tyre isn’t perfectly inflated, the steel wheel can slip on the rubber one.

And then you’re in trouble. Once that slip has begun, there is nothing to stop the machine until it runs out of energy or until the interface between the two wheels dries out and friction returns. But on a steady downgrade, that could be in a mile, or more.

Improved braking

So, to protect track workers and others, not least the excavator driver, something had to be done. Network Rail decided that all class 9b excavators would have to be fitted with a braking system that acts directly on the rail wheels.

This would not only have to apply to new machines, but be retro-fitted to the entire fleet that is used on the railway. And because this is a new requirement, Network Rail would have to pay for it.

Several suppliers were asked to develop a suitable braking system and tender to retrofit the fleet. At the same time an analysis of the exact number of machines used on the railway was undertaken, and a decision was made to fit brakes to about 75% of the hire fleet as these are the core machines that are used by Network Rail on a regular basis. A total of 450 machines were identified as needing upgrading.

Three suppliers were selected – Rexquote, Allan J Hargreaves, and GOS Tool and Engineering Services. All three fitted their system to an excavator, and sent it off to Network Rail for evaluation. After some “tweaking”, all three came up with approved solutions, and The Rail Engineer went off to a snowy Tuxford in North Nottinghamshire to have a look.

Test track

The Rail Innovation and Development Centre (RIDC) at Tuxford is on Network Rail’s High Marnham test track and is about three miles from High Marnham Power Station and ten miles from the Robin Hood Line at Ollerton. A series of sidings and spurs off the single-track line at the RIDC allow vehicles to be tested on gradients, canted track and tight curves without obstructing the main test track.

The first demonstration was of a Readypower Gigarailer which had been fitted with a Rexquote braking system. Although there was snow on the ground, the rails and wheels were dry. The heavy excavator came down a 1 in 25 gradient at 10 miles per hour and stopped just using the original road brakes. It seemed to stop fairly smartly and the flanged wheels locked.

After a few runs at different speeds, the rail wheel brakes were turned on and the tests repeated. Yes, the machine stopped a little quicker, but it didn’t seem too significant.

Spectators were left wondering, to an extent, what all the fuss had been about. The braking arrangement was neat enough, and Rexquote personnel happily answered our questions, but at that stage no-one had really grasped the magnitude of the difference in braking efficiency the new system had.

The rail wheels were fitted with extensions to give more tyre surface area for the road wheels to bear on – which may have contributed to the good performance in road-brake-only trim. The skill of the driver, one of Readypower’s best operators, will also have helped.

The second test was on the flat, and involved a Balfour Beatty medium-weight (22 tonne) excavator fitted with Philmor road-rail gear and brakes by GOS. This one had a special trailer on the back to give a direct readout of stopping distance, including driver reaction time.

As this machine couldn’t switch off its rail brakes (none of them will be able to in service, the two that could were for demonstration purposes only), we couldn’t see a comparative test, but it certainly seemed to stop quite quickly from a variety of speeds.

The brake arrangement was again neat and workman-like. Large disc brakes were fitted inboard of each of the four rail wheels with heavy-duty hydraulic callipers of the type fitted to heavy earthmovers. A protective guard was fitted under each disc so, if the machine should become derailed, the disc itself wouldn’t impact the rail head and be damaged.


However, all debate about the need for the new system was silenced by the third test using a big Colmar T10000, owned by Stobart Rail and fitted with brakes by Allan J Hargreaves. This was the “adverse conditions” test and the rails were constantly wetted by a water spray. To make things worse, washing up liquid was dribbled onto the tops of the rails over a 40-foot length, to simulate the greasy conditions that can be encountered.

The excavator was run back and forth a couple of times to transfer the water and detergent onto the rubber wheels, getting the interface between rubber and steel wheels well and truly covered. Then, with the rail brakes turned off, the excavator came down towards the spectators at 16kph. When it drew level, the driver applied the brakes and – nothing happened!

That’s not quite true. The rubber road wheels stopped going round. But the steel rail wheels didn’t and the complete 28 tonne machine carried on at unabated speed. It eventually stopped some 80 feet further up the track – and that was on a slight upgrade. If it had been braking to avoid hitting someone who had stepped in front, they wouldn’t have stood a chance.

After a few more runs, with the performance being equally poor each time, the new brakes were turned on. Once again the heavy excavator reached a steady speed of about 16kph, the driver applied the brakes, and – it stopped! Without drama, and with the steel wheels just starting to lock, it came to a standstill quite quickly – and about 60 feet earlier than it had before. The difference was startling.

That one test had made believers of all the spectators. Suddenly, the danger of unbraked steel rail wheels was obvious to all, and the improvement using hydraulic disc brakes was very marked.

Everyone gathered round the stationary machine to inspect the new brakes. Network Rail engineers were at pains to point out that all three approved braking systems had a similar performance – the difference on the day was simply due to the different tests being carried out.

They also advised that, if the “adverse conditions” had been applied to the first demonstration on the 1 in 25 gradient, the machine would not have stopped at all and they would have had to go and dig it out of the sand trap at the bottom. From the way they spoke, it had happened.

The big Colmar was then purposely derailed, and everyone could see how the disc protectors worked. But minds were not on that display, but the powerful memory of a 28 tonne excavator sailing majestically down a flat track, road wheels locked, with nothing anyone could do to stop it.


Network Rail plan to have all the core machines converted to rail-wheel-brakes by October. It can’t come soon enough….

Many thanks to Paul Conway, James Allenden, Norman Jordan and their team for organising a very interesting demonstration.