Travel from Poland to Bruges Belgium train or plane?

The Man in Seat 61 is a guide so his pricing might be off (don't know when he makes updates) ... and he usually tells you if there are any tips or tricks you can use for a cheaper fare ... I would go with what the Deutsche Bahn site quotes you for a ticket ... also see if there are any discounts you can get (advance purchase, multiple day pass ... and I believe there is a twin pass where the second person is 50% off but both must travel together) ... if you have the time and funds, would be a nice addition to the trip ... and much more scenic that flying.

Shouldn't you be looking at the Polish Rail site for trains from Poland? There are trains from Warsaw to Berlin. Prices should be listed in zloty.

Going by plane or train depends on basically two factors, your time limit and your travel style and preference. Which takes precedence ? If you prefer flying, check if flights are available from Warsaw to Brussels ,ie, capital to capital.

I prefer going by train, instead flying within Europe. My way would be to take the EC train Warsaw Centralna to Berlin Hbf., day train, 6 hrs or so, spend the day in Berlin , exploring, etc., then take the night train Berlin Hbf to Cologne ( a major junction point), then the train to Brussels, where you can change to Bruges.

From Warsaw to Bruges is about a 20 hour train and bus trip.

Flying LOT Polish Airlines into Brussels is the only airline flying that way. Then Bruges is an hour away on the train.

Think of European train ticket pricing as being similar to airline tickets in the US and elsewhere— they start at the least expensive price when the bargain tix are first released for sale and rise in price the closer you get to the date of travel. DeutscheBahn (bahn.com) puts the heavily-discounted tix up for sale 60 days before the date of travel and they rise in price from then on. For Warsaw to Berlin, the least expensive ticket for the 6-7 hour journey now runs about €35. DeutscheBahn recently announced fares will rise about 5% before the end of the year, so next year it will cost a bit more. If you were to go from Krakow to Prague, the least expensive ticket can be bought on the Czech National Train website www.cd.cz for about US$20 for the 6.5 hour journey. From Prague ( “Praha hl.n.” is Prague’s main train station) to Berlin is a 4.5 hour journey. Many travelers stopover midpoint in Dresden, and the scenery on the way in the Czech Republic alongside the Elbe River is beautiful with castles above the river visible from the train.

From Berlin to Koln (“Cologne”) a fast ICE train takes about 4.5 hours for as little as $20. Again, the DeutscheBahn website 60 days prior to your travel day is the place to get the best deals on the German trains. From Koln to Brussels a DeutscheBahn ICE or Thalys train (Thalys is a private train company with its own website: www.Thalys.com) will take 1:50 with the least expensive tix running about $20 when bought in advance as soon as they go on sale.

This routing is one of the most time-efficient ones to get from either Warsaw or Krakow to Brussels. Flying, of course is quicker, and airfares can be viewed at www.Skyscanner.com Another fun fact about DeutscheBahn is that they will allow you to buy a ticket between two cities and, at no additional cost, stopover at a city in between the two for several hours on the same day. You have to enter the intermediate city you’re stopping at when you buy the ticket. I did this recently on a ticket from Hamburg to Wismar by stopping over for six hours in the Hanseatic League city of Lubeck on the way. You can always take side trips and detours for interesting towns and attractions using this basic route as the framework to help get you started.
Have a great Trip!

I know you have more than one thread, so this post does not completely describe your plans. But we have found, on multiple trips to Germany, that it was difficult to integrate Poland into the trip, even with substantial advance planning. It's also not clear if you mean a train journey as last-minute stops in places where you have no hotel reservation. I personally would not do that. As I may have written before, August is peak European vacation time, and I would also not want to risk being stuck in a hotel without air conditioning in August.

As a halfway measure, you might consider flying from your last Poland city to Cologne, because it is so well-connected to Brussels. But it is fair to say that there is even more to do in Berlin than there is in Cologne. Easier flying plan, you could fly to Frankfurt to take the dayboat on the Rhine (say, from Bingen to St. Goar), and then train to Cologne. But it will be a crowded boat, that I would not do with luggage in August.

https://community.ricksteves.com/travel-forum/germany/can-i-see-enough-of-cologne

If you don't care to read all of my trip report, you might use a search tool to find the sentences that report on the "train" segments in Poland:
https://community.ricksteves.com/travel-forum/trip-reports/four-cities-in-poland-2019

I have not researched the actual rail connections, but it might make more sense to train to Germany from Wroclaw?