These signals are read similarly to the general principles of the three head signal, but with the following variations:
- the signals are only of one colour, usually white or yellow
- the signal lights mimic the shape of the semaphore signal positions
- there is no advanced approach
- the top head is the normal speed signal in most cases
- the second head is medium speed in most cases, except that medium approach is stop over flashing caution
- caution over caution = approach slow
- the diagonal is normally pointing towards the track when the signal is posted on the right hand side of the track
- when the diagonal is pointing away from the track on the top head it indicates block ahead occupied and passenger trains must stop - with a yellow light underneath it means reduce to medium speed and be prepared to stop before the turnout it is connected to
- lower head reverse angle means restricted speed
- stop over yellow light means stop and then proceed at restricted speed - heavy freight don't have to actually stop