Translation:A lot of people are traveling on the long trains.
Comments and a quick google search suggest that buszzal, vonattal & repülőgéppel are ok too.
Although when i think about it travel with something sounds like company. The English by feels better, when i am not allowed to travel in those, but that is then not always applicable.
A lot of people are traveling by (the) long trains. The the has to be removed?
I have no gripe with hajón.
You speak German, right? Hungarian is a lot easier if you keep German grammar in mind. With English it's a drag. :´)
My last comment was a bit imprecise, maybe. I'll try to expand it a little. There are two ways you can use talking about transportation.
One, you can talk about the method. That's what you mostly express in English with "by":
I travel by train. - Vonattal utazom. - Ich reise mit dem Zug.
In English you cannot modify that expression much. You can give it an adjective "travel by long-distance train(s)", but that already sounds a little weird. And you cannot make it specific, so you can't say "travel by the train(s)". That makes it sound like you're running next to the rails. If you want to make it specific, you need to use "with" here: "I travel with long-distance trains" or "I travel with the/this train" (or maybe "I travel using the train").
In Hungarian and German that's no problem. You can say "Távolsági vonattal utazom" or "(Ezzel) a vonattal utazom" just as you can say "Ich reise mit dem Fernzug" or "Ich reise mit diesem Zug". (The indefinite German expression "Ich reise mit dem Zug" interestingly already has the definite article included.)
Two, you can talk about the vehicle itself. You do that when expressing that something is happening in the vehicle. In English you usually use "in" or "on", in Hungarian you mostly use the -n suffix, and in German it's "in" in most cases.
There was a beautiful girl on the bus. - A buszon egy szp lány volt. - Im Bus war ein schönes Mädchen.
Which preposition exactly to use is.. a mess, especially in English. You usually say that something is happening "on the bus" unless you're mentioning a position, then you're standing/sitting "in the bus". It's weird and incosistent, though, and I think you can always say "in" when talking about transportation (unless you have to sit on its surface - like a motorcycle).
I think there are some similarities, preverbs, a little bit more flexible word order, honnan-hol-hova, a lot more still active w-words, vowels, but overall not much is the same.
In German i can say: I traveled with Peter, Anne and the train. Although not necessarily in one sentence. But i think it is obviously apparent why it is also a flawed system (and why it sounds weird in one sentence).
by (means of) is in German actually not "mit" but more precisely "mittels". But i don't think we use it often, although i really like "by" in English and i think "mittels" can be specific too.
Ich reise mittels Fernzug.
Ich reise mittels dieser Fernzüge. Alternatively there is (or was) "vermittels":
Ich reise vermittels dieser Fernzüge.
(damn, mittels needs genitive...no wonder it is dying)
But to sum it up, usually public transport has these prepositions:
German: in or with.
Hungarian: -n or -vel. (stb.)
English: on or by
If you express the mode of transport with "by" in English, you cannot use a definite noun after that. You can say "travelling by train" and maybe "travelling by long trains" to mean that they ride (long) trains to get to their destination. But if you say "travelling by the trains", it sounds like they are moving beside the trains. Instead, you should say "travelling with the trains" or "travelling on the trains".
I get to work by bus. - I get to work with the bus. - I get to work on the bus.