I've never heard the notion before that autists generally like trains, but that might be a thing.

Autism tends to come with hypersensitivity and with too much stimulation, you get sensory overload.
And well, personal experience is that, yeah, trains are the best in that regard. I can just listen to music and then it's basically like being at home.
Buses are less good, because they vibrate more.

And cars are just the worst. Not only do they vibrate, they're also loud (and you can't put in ear buds) and you constantly need to be focused on the street. Assholes will also readily put you into dangerous, even more stressful, situations, if you ever dare to go less than 110% of the speed limit.