NBN EN 15611+A1:2010

This European Standard is applicable to relay valves designed to control the brake cylinder pressure of compressed air brakes fitted to railway vehicles, in association with an air brake distributor valve or other control device, and in response to a change in vehicle load that is either continuously variable or in two stages i.e. empty - loaded.
Relay valves operating with other pressures, in particular the brake pipe pressure, are not included.
This European Standard specifies the requirements for the design, manufacture and testing of relay valves.