Ukraine is huge and has loads of track and trains that gauge, so do Lithuania, Latvia, Estonia, Moldova. There’s even a ukrainian-gauge line running west to Katowice, could make sense to extend it, and make another to Gdansk. Otoh a transversal standard-gauge line connecting Romania to Poland via Chernivtsi and Lviv could also make sense.
Western europe should welcome the technical expertise of Ukraine and Belarus railways, they move a lot, efficiently.
Hey, not so long ago, there was even talk of a canal linking the Dnipro to the Wisla, recreating the old ‘viking rus’ trade-route (although have to consider also impact on wetlands… I recall used to sit next to the IPCC rep from Belarus - he was passionate about methane emissions from wetlands - but suffered from politics …)