I Signals protect the entry to the station. All stations must have an I Signal which also serves as the station limit. All I Signals have at least three lights: red, yellow, and green.

To the right are only some of the very many combinations of aspects and speed boards that can be shown on I signals.

I Signals are part of the station signalling system. The coloured lights tell the driver if the path is clear or not, and the speed board the speed limit if applicable. This can either be in the form of a fixed sign or a light board.

If the signal aspect shows Proceed with Restricted Speed (one yellow and one green) or Proceed Through Station (two green), the speed limit board is valid. It can either be a fixed sign, exactly like a road sign, or it can be a light board. Light boards can show 40, 60, 75 or above, and 40 or under, short path.

In some stations the line diverges, either to a different part of the station or to a different line. This is shown with a diverging I Signal. Both branches of the I Signal can theoretically show the same aspects, but in some locations that is not necessary. Which branch of the signal is lit up is usually determined by a specific switch - hence the yellow light on top may change branch after the switch has changed, but before the path is clear.

SI Signals are Station Block Signals, they split the station approach into two blocks allowing for higher capacity. They look and behave like normal I Signals. More than one SI Signal can be present in a station.