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Sunday, June 16, 2024

Mind the gap!

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Any railway is in constant motion. Not just the trains and the passengers, but the infrastructure too. The track moves in its ballast.

Ballast moves on the trackbed. Even the ground moves – as it dries out in summer, gets soaked from frequent applications of British “weather”, and even suffers from frost heave.

All this movement can cause two types of problem. The quality of the track itself can suffer, and for that reason infrastructure owners such as Network Rail have a robust inspection regime.

The New Measurement Train, inspection cars, sensors mounted on passenger trains and even teams of men are regularly checking the quality of the track.

The other effect of movement is when the actual position of the track changes. Even movement of an inch or two can cause real problems because a railway runs on one thing….


We are not talking about the clearances that caused great upset for the Highland Scots in the eighteenth and nineteenth century, but the small amount of distance between a train and its surroundings.

Maintaining acceptable clearances is vital to the safe operation of any railway.

Sometimes, particularly as railway companies try to get larger and larger trains onto Britain’s crowded network, those clearances can be quite small. And if they go to zero, or even negative, disaster can strike.

The picture of passengers running for cover from a cloud of flying concrete fragments as a train hits a platform edge at 125 mph doesn’t bear thinking about.

But it could happen. There has even been talk of an underground train emerging from a tunnel with the heads of all the rivets on its roof rubbed off. All when clearances disappear.


So every day on the railway there are groups of people in orange coats, hunched over strange-looking machines full of lasers and mirrors and lenses which are perched on top of tripods, surveying the line.

Others have small trolleys packed with GPS sensors, more lasers and complex computer equipment. There are even whole trains, like the Structure Gauging Train, that tend to come out at night (lasers work better in the dark).

All this effort is to measure the position of the tracks accurately with respect to surrounding structures.

How far are the rails from that platform edge, or tunnel portal, or OLE gantry? The surveyors aren’t worried about clearances, those will come later, but actual position and distances.

The entire railway network is effectively broken down into slices, typically at 5 metre intervals. Inside each slice, the distance of the rails from any close structure is recorded, and in some instances checked, and measured again a few months later. Nothing must be left to chance.

And every day, when the measurements have been taken, where do all these surveyors send their data?


Every measurement taken goes, via Network Rail, to the county town of Derbyshire, nestling at the southern end of the Peak District. Here Balfour Beatty Rail maintains the national database for structures and vehicles.

Vehicles are included because the database of structures on its own wouldn’t be much use. Knowing the position of a platform edge is pointless, unless you then factor in the size of the train running past it to see how much clearance there will be.

So the experts at Balfour Beatty Rail work with that data and use it to determine the minimum operational clearances between vehicles and structures. This analysis is undertaken using…


Balfour Beatty Rail’s market leading ClearRoute software was first developed fifteen years ago. It compared the known size and shape of every railway vehicle in the country and correlated it to the position of trackside structures to determine the clearances.

Over the years the programme has been constantly upgraded but recently the Matlock programmers have totally revised and rewritten it to create ClearRoute 2, which is fully compatible with the latest Windows operating systems and has many other new features.

So what does ClearRoute do?

Put simply, it checks whether a train, of known dimensions, can run down a particular piece of track without fouling any structures.

The majority of vehicles in current service on the railway are included in the database, as are all those five-metre slices of infrastructure, and ClearRoute compares a chosen set of vehicles against selected infrastructure.

However, it is not quite as straight forward as that. Every train has a known shape, that’s simple. On curves, the ends of the carriages will swing out one way, and the centre of the carriage will swing the other, so that has to be taken into account. That’s geometry.

Now add the suspension characteristics of the train. How will it sway at speed? What effect do different speeds have? And how will that change if the train is full? Or empty?

And if the track has a cant, how will that affect the suspension, and the sway and tilt of the train?


All these factors can be taken into account using ClearRoute 2. Train manufacturers supply very detailed information on each vehicle and the program uses that and compares it, slice by slice, with the infrastructure database.

The result is a clearance distance between the train and the structure, all around the kinetic envelope of the train.

Going back to basics again, if any clearance is too small, or zero, or even negative, then the train can’t run down that track, at that speed, and in that load condition. However, if the calculations are done again, at a lower speed, what does that show? Is the result the same? Or improved?


ClearRoute 2 can therefore answer several important questions. Can a particular train be safely used on a particular line? Is speed an issue, so that it has to be restricted in certain locations to prevent excessive sway that will affect safety? This is important information for the train operator.

It will also help the infrastructure owner to understand what needs to be done to resolve any problems. How much does the track have to be moved / slewed / lowered / lifted to restore safe clearances?

Will doing that cause other problems, such as interfering with other lines? Or hitting other structures on the other side of the track? The program has a useful “what if?” capability that can be used to simulate changes which can then be assessed.


ClearRoute is licensed to both train operators and Network Rail. The former use it to make sure that fleets can be safely used on both their regular and diversionary routes. And if equipment is moved from one route to another, or cascaded from a different operator, everything can be checked out before it even takes to the tracks.

Freight operators use ClearRoute in a similar way to make sure that their regular freight wagons won’t have any clearance issues. And they can also use it for those awkward loads of irregular shape, and make sure that a selected route is safe to use.

If it isn’t, and the load cannot be transported any other way, then various options can be worked out. Is there an alternative route? If the load will foul a structure to the side of the track can it be taken the wrong way on the adjacent line?

If it fouls the opposing track, can it be transported if that track is blocked to traffic for a short distance? There are many permutations which would take ages to work out using the old-fashioned method of drawings, plans, a pencil and a calculator, but not with ClearRoute.

Underground Overground

One interesting example is the problem of transporting new underground trains from Bombardier’s factory in Derby, and the test track at Old Dalby in Leicestershire, to London.

An underground train is small, but it is also low and, below a conventional frame height, it is relatively wide and out of gauge.

ClearRoute was asked the question, and a path developed that took the unusual route of going via Ashby de la Zouch and Burton on Trent.

Unexpected, but safe and practical. Thanks to the Balfour Beatty Rail’s specialists in Matlock we can now see underground trains out of the rail engineer‘s office window in Leicestershire!

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