Translation:The children get on the long train.
I think that in Hungarian, as in English, you ride "on" the train, not "in" the train, so you get "onto" the train and not "into" it. Similarly with aeroplanes and trams, but unlike cars.
Yes, you travel "on" the train but, nevertheless, you can "beszállni a vonatba". It is not bad at all. You can also do that with buses and aeroplanes, etc., if you want. With cars, that is your only choice. With trams, I would stick with "felszállni", for some reason.
Interesting that you say that: I would say "beszáll az autóba" but not "beszáll a vonatba".
Yes, you would normally say "felszáll a vonatra", but I can imagine "beszáll a vonatba", as well. It is not bad. What would be bad is "felszáll az autóra".
(you can search for "beszáll a vonatba").
"Beszáll a vonatba" makes much more sense in modern railway stations where the platform is level with the floor of the train cars, so you just walk on board.
yes, as I said, to my ears, "beszáll a vonatba" sounds weird, but "felszáll az autóra" would definitely be wrong... I guess it has something to do with the physical movement...