A control command and signalling system does not brake, i.e. it is not responsible for the braking system of the train, which will do the actual job. ETCS (and also some elaborated legacy systems) supervises both the position and speed of the train to ensure they continuously remain within the allowed speed and distance limits, and -if necessary- it will command the intervention of the braking system to avoid any risk of the train exceeding those limits.
To do so, the ETCS onboard computer must predict the decrease of the train speed in the future, from a mathematical model of the train braking dynamics and of the track characteristics ahead. This prediction of the speed decrease versus distance is called a braking curve.
From this prediction, the ETCS onboard computer calculates in real time braking distances, which will also be used to assist the train driver and to allow him/her to drive comfortably, by maintaining the speed of the train within the appropriate limits.
The ETCS baseline 2 specifications lay down the basic principles for the braking curves and the associated information displayed to the driver, but there is still no harmonised method/algorithm to compute them.
The convergence towards a stable specification of the ETCS braking curve functionality has now been achieved and is reflected in the ETCS baseline 3 specifications.
The ETCS braking curves algorithm consists of a complex set of equations. The Agency has developed a tool, which allows predetermining all the braking distances, as they would be computed by the ETCS on-board equipment. The tool allows capturing all train data (including the correction factor K_dryrst obtained from the Monte Carlo calculation) and trackside, which are relevant to the braking curve calculation and displaying graphically the braking distances so calculated.