Nowdays at the trains last compartment behind the X so would have written so that it know is that this train last box.
If any of the train rear compartment X do not be written, that it would mean that this train compartment on the way ceased to be show on the master station to the railway informed so that any train of the cans of the accident.
If the night be so X instead of the rear of the train a red light occurs from the same can be detected that the train of the last box.