I got trainbow on a scenario on saturday and later on when i finished a timetable run that terminated at rochester, the next train pulling in was trainbow so i drove it to st pancras.
I got it this morning on a late night run to St Pancras. I've not been trying to get it, it's only when I went to the outside camera that I noticed, I then went back in the cab and drove on as normal. It's a train with a rainbow on it, it looks nice and yeah, that's it. Some people seem to be making out like it's the be-all and end-all.