Translation:The children get on the long train.
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.