Rail101 - Don't let track circuit signalling blow your fuse

Robust signalling of railways is vital for efficient and safe operation. Track circuits are often used to provide detection of trains in segments of track. There are other signalling methods but this #rail101 article is going to talk about just this one in a very basic way (I'm not a signaller!)

Track circuit signalling uses the rails to conduct small electrical currents around a section of track. Lengths of track are broken up into sections (blocks) with insulated joints (usually) between each. When a rail vehicle enters that section of track the wheels short circuit (shunt) the electrical path (via the axle between the two wheels), and this triggers the signalling system to mark that section of track as occupied by a train and provide inputs to signalling systems that control the safe operation of the track.

Images from Matt Johnson (CC BY-NC-SA 2.0)

Where track is empty the relay (2) is energised by the power supply and the signalling system knows that the track is unoccupied. When a train passes into the section it bypasses the relay - so it becomes de-energised (4). This lets the signalling system knows that section of track is now occupied.

There are many variations on this theme , with AC and DC circuits and even ones where there is no insulated gap between blocks but the basic principle remains that the vehicle axle provides the shunt (short circuit) across the rails to trigger the signalling system that the section of track is occupied and therefore provide inputs into signalling systems to keep trains safe.

The system, as with all signalling systems, is designed to fail-safe. If the circuit is not completed the signals turn red. This can deliver a way of detecting broken rails if the two rail broken ends are not in electrical contact with each other, but of course not all broken rails end up in two pieces.

One potential drawback of track circuit signalling comes where the rail head is rusted/oxidised or contaminated. If the train axles do not make a good electrical contact across the rails then the signalling system gets upset and as these systems are designed to fail safe, the signals turn red. Lightly trafficked tracks are most prone to this type of issue, (as regular passage of traffic over the rails usually leads to a polished shiny surface). Other problem-prone areas are cross-overs, passing loops and station termini. To avoid this loss of shunt sometimes stainless steel weld beads are applied to the rail head to create a better electrical contact.

This stainless steel weld bead on the surface helps ensure electrical contact between train and track but does have a number of downsides.

• Costly and labour intensive to apply the welds on site.
• Quality/consistency is often not great - they often spall out as seen above
• They provide a "rumble-strip" surface which generates noise and vibration and necessitates speed limits which can be an issue in some locations

In my view a far more elegant solution is to use a rail manufactured with an integral stainless steel surface. This solution has been used in France for over 20 years from sidings to TGV routes. The smooth stainless steel surface resists oxidation and is installed using standard processes making it simple to use, effective and introduces no speed limits or extra noise/vibration. It is called Sogenox.

The stainless steel layer on the rail is integral to the rail as the rail is manufactured with this layer already in place. The stainless steel layer is metallurgically bonded to the rail crown before the rail is produced so there is no danger of it becoming detached in use. Sogenox rails are produced to the same flatness/straightness tolerances as new rails, so there is no need for extra speed limits an no extra noise or vibration is generated. Standard welding procedures are used used to ensure simple and rapid installation making it a simple fix for a tricky problem.

As I try to make my articles both educational and balanced, I tried to find a good summary of the types of and alternatives to track circuit signalling. There is a great article from Paul Darlington which discusses many of the types and some issues here (~15 minute read).

I hope you've enjoyed this article and found it useful. Follow the #rail101 tag for more like it and follow me for more rail related content.

Drop me a comment below if this has been useful, you think I've missed something, or of course you need any rail advice. Thanks for reading.