OH is incontinent and needs to be near a loo. He also gets bored easily and starts drumming his hands on the table. I remember a stressful journey back from Edinburgh where our quiet carriage filled up more and more and got louder until people were standing in the gangway blocking access to the toilet. I wowed from then on always to book First Class if it was affordable.
In contrast, when we went to the Vitalise Centre for holiday in March, we had an uneventful 3 hour trip but I confess that I don't really look forward to train travel anymore and won't do it for the fun of it anymore unless the journey isn't very long.
Book seats near the toilet and take lots of distractions with you like simple games or puzzles,plus snacks. Basically, plan as if you were going away with a toddler.