For Germans: Es sind keine Tische IN den Straßenbahnen. :-)

Who would expect tables on a tram anyway?

is this on the trams as in on top of the trams, on the trams as in inside the trams, or both?

tramway should be accepted. reported

If it were on the tram (not plural), would it start A villamoson, rather than a villamosokon?

In the US we use the word streetcars--this should be accepted.

Is "villamosokon" really something a Hungarian would say?
I'd expect villamosban, unless we talk about those tables being on the roof of the tram???

There are no, there are not, or there aren't should be valid answers.

Is this statement general? I'm still not sure in which case we should use "a" and in which not... would the sentence without "a" in the front be wrong?

ON supposes being outside the object, on its surface - this is highly unlikely in case of a table and a street car. So IN should be the right solution here