I can't be 100% sure if this was actually what we did, it was around 1987/88 and I was only ten/eleven years old at the time, but I seem to recall us doing the sleeper from Euston to Aberdeen, then heading straight back to Kings Cross on the first HST of the day, then another tram straight back to Newcastle and then another to Plymouth. I believe it was part of an effort to win the Rail Riders annual mileage competition which was running at the time (we didn't lol).
Maybe somebody can confirm if that move was possible at the time or whether I'm imagining it?