"A lot of people are traveling on the long trains."
Translation:A hosszú vonatokon sok ember utazik.
It sounds a bit unnatural, but it's not wrong. It should probably be accepted.
The reason it sounds a bit strange is that the position in the sentence just before the verb is where the emphasis usually lies, so your variant emphasizes that a lot of people travel on the long trains even though some other mode of transportation may have been expected. This may make sense in the context of a conversation, but out of context it's unnatural.