I'll normally show my saveaway to the driver of a Merseyside rail rep as they are normally Arriva buses, so the driver will know what my ticket is. They normally just wave people on in general.
During the loop track renewal, tickets were checked by Merseyrail RPIs (normally-sometimes just regular staff) on boarding the bus, making sure people knew which bus they were on at the time.
I boarded a rail rep at Salford Crescent a few years ago to Man Vic. The driver asked us if we had tickets, I went to get it and was promtly told 'I'm not bothered either way really-but they're fining people at the other end...' ('Penalty Fakes' of course in this case).
Slightly off topic, but I never managed to get a clear answer. Do the Merseyrail Byelaws apply to replacement buses...? (Feet on seats, no drinking ect) I asked a Byelaw Enforcement officer and the reply was pretty much 'That's a good question!'
I wonder if GTR would have been brave enough to PF anyone during the Redhill shambles of a few weeks ago...
