I can only half-remember the details now, but I think that looking at Virgin/Avanti trains on Open Train Times there were some apparently duplicated schedules, one with a '1Z--' headcode. I was given to understand that two schedules were entered for 'double' Voyagers, for reservation reasons, but can anyone confirm or explain further?
The use of WTT 1Zxx schedules on Avanti is strictly limited to trains that divide or join en route, e.g. the 1823 Euston-Shrewsbury is 9J38:
Realtime Trains provides live realtime running information for the Great British railway network using open data.
www.realtimetrains.co.uk
But there is a duplicate 1Z38 1823 Euston-Wolverhampton in exactly the same path:
Realtime Trains provides live realtime running information for the Great British railway network using open data.
www.realtimetrains.co.uk
As I understand it, this is a fudge in order to get the booking system to allocate seats in a particular way: without it, someone travelling from Euston to Birmingham could be allocated a seat in the portion going to Shrewsbury, therefore taking up capacity that could be used by someone going to Shrewsbury. By adding in the 1Zxx schedule, the Euston-Birmingham passenger may be allocated to 1Z38, not 9J38, and thus have a seat reservation for the portion terminating at Wolverhampton.
This is specifically for trains that gain or lose carriages en route, not ones that have two portions with different origins or destinations - e.g. it is
not necessary for the 1D92 1810 Euston-Holyhead, which splits at Chester but whose rear portion is then 1J32 to Wrexham.
I don't know why the other InterCity TOCs don't seem to need to do the same, though. I have a feeling XC only ever put reservations in one unit, even when something's booked a double Voyager, but I'm not sure.