"The children get on the long train."

Translation: A gyerekek felszállnak a hosszú vonatra.

[bnyugat]

In the previous sentence felul (with its diacritical marks) was another word for 'getting on' the train, so why isn't it accepted here???

[vvsey]

It probably should be. A tiny nuance could be whether you can normally expect to be sitting in the vehicle. On an airplane, you will definitely sit. So you would more frequently use "felül". But this is just a minor difference. Also, "felszállni" emphasizes the actual boarding, while "felülni" is closer to indicating a mode of transportation. Again, the difference is small.