In the 70's I took my motorbike(s) on the train numerous times, including the sleeper from Kings Cross and then Euston. I took them to Oban, Fort William, and Mallaig, Kyle of Lochalsh and Wick. It was no trouble in those days. I have also taken it to Cardiff to tour the Valleys. I also went to Stockport a few times when looking over the old GCR at Woodhead and Penistone etc. I would take the train one way and cover as many miles as I could making my way back home.
A few times whiel returning south from Scotland I would left the sleeper at Rugby and spend the day returning home visiting the station sites on the Ex GCR London Extension. There was still a fair bit to be seen in those days.
Over a period things changed and you were just allowed enough fuel to get to the nearest garage at your destination, then slowly the cost rose considerably then they were totally refused.
During my 6 years at Marylebone Depot I took the motorbike on HST's between Didcot and Paddington, mostly just one eway as I would either need the bike to get to work or drive home after missing the last train home. Bank Holidays was always difficult one way or the other.
My first bike was a150cc Lambretta, then a 125cc twin engine Suzuki and the final one a 160cc Suzuki GT. I never experienced any problems and always had the bike well tied up to the wall of the guards vehicle.
When the HST's came in I thought they wouldn't accept motorbikes, but as I say, it was over a period before these changes came about. However by the time I stopped taking my motorbike on the trains the cost of transporting them became very expensive and would have been cheaper to drive all the way to Wick than what the fare was. If I recall correctly it started at just over round £2 and finished up at around £80.
I have many fond memories of many great trips done that way.