I think it is a reasonable thing to do. She won't be able to sit in the buggy like she does on a bus, and having her on my lap for the entire journey would be a nightmare. In her own seat she could at least look at books or do some colouring, and, if she does want to sit on me, I will have somewhere to put all of her stuff.
It's been a long time since I travelled by train and all I remember is how cramped it was. This seemed like a good idea when I was buying the tickets, but everyone else here seems to think I'm being silly. Am I?