And why not accept this translation as well? "Hungarians are travelling on these trains, whereas Belgians are on those."
Is it indispensable to repeat the verb "travel" in the second part of the English phrase?
No; "Hungarians travel on these trains and Belgians on those" sounds fine to me and should also be accepted. Report it if it isn't. (I'd prefer "...on those ones.", though.)
Hungarian people, or Belgian people not accepted. Are we talking about Windmar Belgians?
Why is this one wrong? On these trains Hungarians are travelling, and on those Belgians.