Episode: Shapes
The very first episode of Rainbow featured a group of children travelling on London Transport RM1242 (242CLT) on route 281:
Seen along the way is this Plaxton-bodied coach:
Curiously, though, the last shot is not of the RM, but of an RT on route 72!
"I recall that a mid-1970s edition featured the music group singing a song on the rear platform of a London Transport AEC Regent RT. On another edition - or it may have been the same edition - the programme closed with shots of London Transport buses running in service, which I think included an AEC Merlin and an RF.The programme may have been devoted to showing children the work of a bus conductor,or something similar." (thanks Alan Sinclair)