By taking a pressure reading at a particular port on a multiple-unit braking valve, e.g. the MU-2-A valve, and comparing such a reading to a main reservoir pressure, one can identify whether the locomotive in which the valve is situated is in trailing mode or leading mode. In particular, it was found that the pressure on port 63 of the MU-2-A valve is indicative of which mode the locomotive is in since port 63 is exhausted to atmosphere in lead mode whereas it is at or near the pressure in the main reservoir which is readily distinguishable from atmosphere.