RSIDs are in the schedule data; they're the combination of the TOC code (e.g. GW) and then a four digit code, which is often numbered sequentially for departures on a given route. There's then two final digits which indicate the portion for multi-portion trains, but which are normally otherwise just 00 and hence not showed in most places.Thanks! It's good to confirm the maximum length of the data field.
Where do the RSIDs come from? I feel like someone's answered this before, but it's not something usually displayed, well, anywhere. I think some booking engines do display an identifier which looks similar, but I don't know if that's the RSID or something internal to that site.
RSIDs are shown on some timetable printouts (e.g. Network Rail's PDF WTTs, I believe?) and it's also shown on Real Time Trains and the like. They're also shown on the side of some trains, e.g. XC Voyagers.