Hi NSWGR_46Class
I am pretty sure now that the numbering issue is a TS12 feature. A little while ago I started with a new blank map, laid a length of track, added a 36, tender and a couple of wagons. I added a number and headboard to the loco in loco properties (bottom right of course, not left as I said earlier). Very easy so far. Then I clicked the consist button, then get consist button( that one is bottom left) and named the consist and saved it. I then laid another piece of track and called up the newly named consist for it. No numbers on the loco and I hadn't even left the session. I Deleted both consists, saved, exit game, startgame again a little while later, loaded map and session and placed the consist on track. Same deal -- no numbers or headboard. So I don't think it is anything to do with your scripting (or anything to do with Quick portal manager either).
If you like I can send you the route and session if you think I may have done something else wrong.
Bye for now,
Tom