One of the worst trips I took was on the dreaded Southampton-Bristol service with 3 carriages. Bike section was so full of people I had to balance the bike on the rear tire whilst avoiding ramming the bars into peoples heads, if we had to brake sharp it would have head kebabs all round.
After a few trips realised this was the norm at peak weekend times so now hire a car, 40 quid extra well spent. The state of differing cross network policies/archaic systems and limited places make us the laughing stock of Europe, bouncing around 15 years ago in the guards van was way more enjoyable!

