+1 for Gdansk and Elblag from me too, We did it about 10 years ago, fascinating, stayed a couple fo days in Elblag, went to Malbork castle as a day trip from Gdansktoo. Book here:
https://zegluga.com.pl/en/oferta/the-route-through-the-elblag-canal-ostroda-milomlyn/
The only sad thing was that you couldn't hang around to see one of the waterwheels which power the inclined planes*, also it lost time loading lunches for a big party of disabled people so was late at the top and there wasn't time to do the museum or buy food before jumping on the coach back to base.
(*There are lots of videos on YouTube though.)
You can book the boat in both directions if you like, rather than returning by coach, although I seem to remember having to do it as two separate bookings. I saw the waterwheels by doing a land-based trip the day before -- I got the train to Pasłęk, from where it was a bit more than an hour's brisk walk along a fairly quiet road to reach the canal near the middle one of the 5 inclined planes. There's a path alongside the canal to the top incline, from where it was a similar walk back to the station at Nowa Wieś (one stop south of Pasłęk). Check the timetable, though, as trains aren't that frequent (especially at Nowa Wieś). There may be local buses in the area, but I didn't have much idea how to find out about them.
On the OP's original question, Kraków to Zakopane is quite a scenic journey through the Carpathian foothills; when I was last there in 2016 the stopping train on which I travelled was a communist-era e.m.u., but I don't know whether that's still the case. En route at Chabówka there's a railway museum, with steam excursions in the summer:
http://skansenchabowka.pl/ (although I haven't visited it myself).
I don't know whether the OP is interested in narrow gauge lines; this English-language website listing them:
https://www.narrowrail.net/poland/ hasn't been updated for several years, but it might be a useful starting point. I think all are now tourist operations (not necessarily steam) rather than public transport. I think the Bieszczady line is meant to be quite scenic, although I haven't been there. As the OP has posted on the Wolsztyn thread I presume they have seen my mention of the Środa line there.
Just across the border in Slovakia, the narrow gauge Tatra Electric Railway is stil part of the public transport network (with modern stock). If you travel from Poland to Slovakia by the bus from Zakopane, instead of staying on the bus to Poprad you can get off at Tatranská Lomnica and transfer to the n.g. railway (or the standard gauge branch that also terminates there).
