In its simplest definition, the trackside signals are instead placed inside the driver's cab of the train. This means that the train is not limited by the diadvantages of block signalling, where all trains must adhere to a one-size-fits-all and to a worst-case-scenario method of signalling around the geogrpahical features.

For cab signalling, each train receives independant information from the trackside signalling system and transforms the information into actions for the driver to obey. Cab signalling makes It possible to have non-geographical or moving blocks and for trains to respond to their unique characteristics of length, speed and mass.