Translation:Waiting for a train is not traveling.
It's not very natural. Your construction is using a false subject "it" to postpone the real subject "waiting for a train". We do this sometimes, for example when the subject is an infinitive - "It's not nice to be kept waiting", but the natural position for a gerund subject ("waiting ...") is in normal subject position, at the beginning of the sentence.
Fair enough but where do you draw the line between being overly pedantic about English and just saying - yeah, that sounds about right. Because in every day conversation if you were to drop 'I'm waiting on a train' absolutely nobody would bat an eyelid at that, nor would they if you said 'I'm waiting for a train'. In short - both are perfectly fine in English and we're splitting hairs here.
I apologize... as it turns out, at some point we decided to accept "to wait on" despite all doubts.
The problem is that "waiting on a train that is not travelling" doesn't work without a comma, since "that" splits it into two seperate clauses. In the Polish translation it's still one clause. That is a significant structural change to the sentence, so here we decided to draw a line.
Rule of thumb: If the Polish sentence starts with "To (jest/są)" you can translate it with it is/this is/that is/these are/those are. But if it's somewhere in the middle and the sentence doesn't have a comma, just use the copula "to be".
It was a different case, after all this is a gerund, so it's a type of a noun.
I see this was "Oni rozmawiali o podróżowaniu", so "about travelling" -> Locative.
This is a simple "X is (not) Y" sentence, so it's either "to nie podróżowanie" (Nominative) or "nie jest podróżowaniem" (Instrumental).