Modelling Time Locks on Switches

Time Locks – An Introduction

In the real world, manual switches within signalled territory are protected by devices called “time locks”.  The purpose of these is to prevent a switch from being opened in the face of an approaching train.  When the conductor wants to open the switch, he unlocks it and starts the timer running (how this is done depends on the model of time lock).  The time delay gives any train too close to stop – or sometimes too close to even see a restricting signal – time to safely pass over the switch before the points are changed.  It also triggers the signal system to display restricting aspects around the block, so trains that are further out are alerted to the presence of an open switch.

Once a programmed amount of time has passed, the timer indicates to the user that it has expired (often by a white or green light) and then releases a locking mechanism that allows the points to be moved manually.   (This is commonly done with a locking pin through the throwbar that is retracted, but there are other mechanisms.)

Time locks aren’t just a good idea – they’re required by law here in the US.  Under 49 CFR 236.207, either approach or time locking is required of manual switches in signalled territory.

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