Four Aspect Railway signal sequence

I thought this might be of interest to any railway enthusiasts out there. It is a sequence showing the order in which railway signals here in the UK operate. This signal is in the National Railway Museum in York and is on a timer that switches the lamps on and off in the correct order as a demonstration.

From the left we have:

Red - Stop (self-explanatory, really..!)
Yellow - Caution. The driver may pass this signal, the next one will probably be at red (unless the train in front has passed the next signal in advance of it) so it's probably a good idea to start slowing down..!
Double Yellow - Preliminary Caution. This signal may also be passed, the next signal will probably be at Caution, be prepared.
Green - Proceed. The next signal will be either green or double yellow.

If the signal is on the approach to one or more diverging routes, five white lights above the signal to give the direction of the signalled route will be illuminated, except if the route is straight ahead. If on the approach to a station where there are several platforms available, an indication of the platform number the train will enter is usually shown on an additional dot matrix display.

In addition, on high speed routes there may be two additional aspects:

Flashing yellow - warns that a lower-speed diverging route is set at a high speed turnout.
Flashing double yellow - indicates that the next signal is showing flashing yellow.

Some signals (mainly off the busier main lines) have only three aspects, the double yellow being omitted. There are also certain circumstances in which only two aspects, either yellow/green, or possibly red/yellow, may be displayed, but these are relatively rare and only found on very quiet lines.

An alarm sounds in the driver's cab on approach for all signal aspects except the green proceed one. This must be acknowledged by the driver pressing a button, or the emergency brake will be automatically applied. For the proceed aspect, a bell sounds and no action needs to be taken.

This style of signal is rapidly being replaced, unfortunately, but there are still significant numbers of these out there to be seen :-)