Emotional argument :
I would appreciate such a car even in solo mode if it can be immersive and let me watch out from the window (I'm not driving). A bit like the introduction scene. That scene and the feeling of going outside were awesome.
Gameplay argument :
Jumping in the train often mess up its schedule (AI disabled). It doesn't break it, no bug here, only the slightest delay can affect the production rate my of factory and cause power spikes. So I have to drive exactly like the AI... I'd rather just be a passenger sometimes.
Jumping on the train... Is not really comfy. I can fall and die easily on some bridges, and the next train won't be there before 10 min ; plus I often get a shaking effect as if I was falling then standing ~20 times per second when I stand on wagons. And I can't go through tunnels like this... If I collide with another train I fall too.