Translation:We sit on the train and look back at the river.
onto the river was rejected. sometimes onto is mandatory, sometimes it is prohibited. Good luck!
Anyway, I think the right idiom here is "look at" the river. Or is it just in the direction of the river?
It should be "look at". I guess "look to" would be translated with a folyóhoz.
So true. It seems to depend upon the whim of the developer as to the literal or metaphorical/idiomatic toss of the coin. In American idiom I would never sit "on" a train, I would always sit "in" a train. To sit on makes me think of riding on top of the train with the wind in my hair. The literal "on" was preferred. In the second half of the sentence we literally look "onto" the river but the idiomatic "at" is preferred.