I found that the Scotrail website allows cycle reservations to be booked at the same time as the tickets. Whenever I have the chance to fill in a survey on a train website I make this point.
I only do 5 - 6 Audax events a year. I used to drive to most of them, but now the majority I ride to the start or take the train, which avoids a tired drive home afterwards.
Most obscure train station is probably Caersws, which I used as a start/finish station for the Cambrian 4C. Oddly enough I had two seats to myself on the way back
It feels expensive (£124 for the round trip to Newcastle from Basingstoke for the Border Raid even when booking 12 weeks in advance), but that's 20p a mile, which is about the fuel cost for my car, not taking into account wear and tear on the vehicle and a minimum of 6 hours of sheer hell (probably more like 10) sitting in summer Friday traffic.
If you are travelling Cross Country between Birmingham or further north and Reading or further south, split your fare at Banbury - it will half the rail fare. Makes cycle reservations a faff, but you will save a fortune.