This is a problem that has existed since GTA III. Whenever you get on a train/metro, it just turns to a cutscene. Same with the cable car.
They're completely pointless elements in the game if you can't do anything in them.
It would be so cool to be running away from the police, get on a train, and cause more havoc whilst its moving. Same with the cable car, perhaps being able to shoot out of the window whilst a helicopter is after you.
Is there a technical reason why this is a feature that R* are unable to implement? Because it seems like it would've been an obvious improvement to me...