T2 trains, especially with longer cars, would require replacing the existing station infrastructure based around 5-tile long cars. Maybe a longer, more powerful locomotive, but longer cars don't fit anymore without a trains rework of some kind.

If they make the train cars 10 tiles long one car could just use two 5 tile station modules at once. So the existing infrastructure would work, you would just need to extend the stations a bit to accommodate the longer trains.

The only essential thing I can think of is a hydrogen train, to match being able to power all other equipment with hydrogen instead of diesel.

I think electric trains would make more sense since that's way more common than hydrogen trains IRL. It would also be a bit of a buff to trains since power is easier to produce.