ETCS Level 1 is a cab signalling system that can be superimposed onto an existing signalling system.

Signal aspects are picked up from trackside signals by adapters and telegram coders (LEU) and – together with the route data – transmitted to the vehicle as a movement authority via Eurobalises. The on-board computer continually processes and monitors the data to work out the maximum permitted speed and the braking curve.

Due to the 'spot transmission' of data, the train first has to travel over the relevant Eurobalise in order to obtain the next movement authority. With the installation of additional Eurobalises (infill balises) or a EuroLoop between the distant and the main signal, the new proceed aspect is transmitted more frequently, or depending on the size of the euroloop, quasi-continuously. The EuroLoop is an extension of the Eurobalise over a larger distance, allowing data to be transmitted more or less continuously to the vehicle.