I don't know who I'd want to travel with if I was thinking of actually nice companions, but I can think of many who might be entertainingly terrible! The first to come to my mind are Feng Xiao and Cui Buqu.

I can't remember any train related arcs off the top of my head. I like trains as a setting and they can be good for mystery/horror, but I guess none actually stood out to me in the novels I've read!