Translation:The workers travel by tram and by bus.
No, you're not missing anything, "on" would be just as good as "by" here.
Fwiw, there's another Hungarian ending that's used with modes of transportation that would feel like a more natural translation of "by" to me (villamossal és busszal) but it hasn't been in the lessons so far.
It seems correct to me. If I were to say a sentence like this, I think I'd probably say "The workers travel by tram and by bus." The only situations I can think of where I would use the plural would involve specific trams and buses, but then that would probably involve the definite article.
"Tram" is generally used across Europe. But yes, it's a streetcar in Canada and I don't know any Canadians who call it a tram.
"The workers travel by streetcar and by bus." (Reported 2019-08--24)