Tollerton Control

If a short, fast train proceeds over two adjacent track circuits of different designs and - critically - response speeds, the train could momentarily disappear until the second track circuit has registered occupancy. This can cause route locking to drop, resulting in pointwork ahead of the train being unlocked. This is obviously a very undesirable situation so Tollerton Control was invented to deal with this situation.

This is achieved by one of two methods:

• By delaying the clearance of the first track circuit for a longer time than the occupancy of the second track circuit (more common)
• By not allowing the first track circuit to register clear until the second track circuit has registered occupied (less common as it's complicated and prone to problems during perturbed operation).

With the masses of axle counters now out there, this has happened with axle counter heads, due to the exit of one axle counter section being fed to a different evaluator than the entering of another axle counter section. The messages from the different evaluators have not necessarily arrived in the correct sequence back to the interlocking, causing the same effect as above.

Last edited by GeoffM on 15/09/2016 at 03:01