Delete the road between the two cities and they’ll be forced to use your trains! muahahaha!
When upgrading roads between cities always, always, make a bus lane if you have a route there – it makes a colossal difference in the travel times. As an added bonus any trucks that use the road will also use the bus lane.
As to the OPs question, yes I can’t make town bus or tram routes profitable either, town to town is fine but if they don’t wanna use my busses or trams – sod em, they can walk.