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Presumably the loco has been run round and is at the other end of the train, due to a failure of the DVT or the control mechanisms from it. Not sure why there is an external tail light on the mk4 though, it should have one built in.
They're hardly ever used so I'd not be surprised if one or both of the lamps wasn't working. Even if they were working they're usually filthy so whoever was setting it up may well have preferred to hang a lamp rather than get filthy themselves cleaning them up.