Traditionally, it was red & found at the end of a freight train
[DOC] [TLE] Reinventing the train: Ideas coming down the track - The Economist [PAR] Jun 1, 2013 ... Transport: New train technologies are less visible and spread less ... for a 
locomotive operator at the end of the train, notes Carl Van Dyke of ... his freight 
train, its emergency brakes screeching, headed towards a ...