Assuming that the leading loco was at fault as above, would they be allowed to move only the two loco's forward, as the same fault would still be present after they were uncoupled from the train coaches? I wonder if a third loco would be needed with correct working equipment to lead in that situation?
That is not a problem since PTC is not really necessary to operate anything within a yard at "yard" speeds. There are procedures in place for operating without horn and headlight in yard that can be followed.
I was wondering more with reference to the original proposition above, where the lead loco has "failed", possibly with the PTC or the other problems you mentioned, on the main line, and the two locos need to be moved on that main line, to a place they can be "swapped over"? Will a third loco need to head the procession?
My understanding is that trains can operate with some restrictions with PTC out in the lead unit. They are not allowed to depart a terminal station with PTC out in the lead unit. I am not sure what the rules are for headlight and horn. Worst case they'd have to wait for a rescue locomotive coming in from the right direction to pull the train off the main into a siding.
In the UK most of the system replies on AWS enhanced with TPWS at key locations such as junctions where there is a danger of SPADs (signal passed at danger). I believe the rule is that they can operate up to 40 mph if the equipment has failed. Note that AWS only enforces a driver acknowledging a signal that is other than green, he/she may still proceed at any speed after the ack, unless TPWS is provided which does enforce a speed restriction.
