If a loco is dead hauled the battery switches are normally isolated so therefore no built in tail lamps will work that is the answer to one question.
If a multiple unit has a portable tail lamp this is normally used if an engine shuts down and there is no remote supply circuit to feed power to the rear coach the portable is put in place in case the batteries go dead which will cause the tail lamps to go out.
Tail lamps are a necessity especially in AB worked areas. Another rule is if a train needs to be stopped in an emergency and the unit is not fitted with hazard lights (alternating head lamps flashing) the a driver will switch on and then off their tail lamps to stop an advancing train.
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If I see a train approaching without a head lamp I will signal to the driver that they have not got a head lamp, If I see a train without tail lamps I will buzz my guard up to call the guard on the said train to switch them on. No need to call the signaller, control, the driver manager, the directors of the company or the RIAB
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Class 175's have an indication that you have tail lamps on the rear of the train on the TMS