Anything going to clear the line should be 1Z99 -
An interesting question - I wondered about that when I wrote my first response! In days gone by, where duplicated headcodes were far more common (all locos returning to a depot, for example, would often be allocated the same headcode, specific to that depot), it'd be easier to accept. I can see the potential for confusion though, with two or three 1Z99s all needing to be talked past signals!True, but what happens when you have multiple rescue locos going to the same incident? I assume they can't be all 1Z99s can they?
Anything going to clear the line should be 1Z99