Much of Poland is low lying and/or flat so is not very scenic in the usual railway sense of the word. I once travelled on a very slow diesel hauled cross country train in the border country east of Warsaw and it was incredibly boring even for a diesel haulage enthusiast. The clue to this is the geographical fact that if you were able to see east far enough when standing on the mountain hilltop in the Peak District, the next highest point is the Ural Mountains in Russia.
If you look at a physical map of Europe and follow the latitude 53.5 (approx!) line east from the UK you will see what I mean. All the places in between are low lying (Netherlands, northern Germany, Poland, Belarus, Russia.
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The northern regions are somewhat hilly, and flat, fertile farmlands dominate the Central Lowlands.
Moving south, the land rises into hilly uplands that front the Sudetic and Carpathian Mountain ranges. The tallest peaks are in the Tatra Mountains. The highest point is Rysy at 8,187 ft. (2,499m)
The Oder, Vistula and Warta are the country's major rivers. Numerous small lakes dot the far northeast
As such the scenic lines (in the conventional sense – ie with hills or mountains) in Poland are restricted to the Poland southern border strip as I’m not aware of any lines that hug Poland’s major rivers. I’m not sure which lines are still open to passengers, but I some years ago I travelled on Zagorz – Lupkow (Kriegslok 2-10-0 steam hauled!) and Klodzko – Kudowa Zdroj and both were very scenic.
Otherwise anything along the southern edge of Poland is theoretically scenic – such as around Jelenia Gora, Bielsko Biala, Nowy Sacz and Zakopane
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