It’s not a “complication”. Rail Baltica Global Project is already integrating rail services through Poland and the Baltic States (and, by extension, Finland) to match the 4 ft 8 1/2 inch gauge used throughout the rest of the EU. By the time the tunnel is complete and the trains start to run through it they will be able to run the length and breath of the entire EU. Goods from Seville could be freighted directly to Helsinki and vice versa.
Meanwhile we’re pushing on with wasting billions destroying the countryside to knock twenty minutes off the trip from London to Manchester for commuters who are probably going to be working from home from now on anyway.