The correct answer is: B. point lock
A point lock is a mechanical device used to ensure that the route cannot be changed while the train is on the point even after putting back the signal. It is a safety device that prevents the train from derailing.
A detector is a device that detects the presence of a train. It is used to prevent collisions between trains.
A lock bar is a device that is used to lock the points in position. It is used to prevent the points from being moved while the train is on the track.
A stretcher bar is a device that is used to stretch the points. It is used to ensure that the points are aligned properly.
Here is a diagram of a point lock:
The point lock consists of a lever, a pawl, and a ratchet. The lever is connected to the point rod, which controls the movement of the points. The pawl is engaged with the ratchet, which prevents the lever from being moved unless the pawl is disengaged. The ratchet is turned by the train’s wheels, which disengages the pawl and allows the lever to move the points.
When the train is on the point, the lever is in the “locked” position. This prevents the points from being moved, even if the signal is put back. This ensures that the train will not derail.
When the train is clear of the point, the lever is in the “unlocked” position. This allows the points to be moved, if necessary.