Side note: This gauge difference worked in Ukraine's favor in one respect. Russia's military logistics infrastructure is heavily train-based, but Russia's trains had to stop at the Ukrainian border because of the gauge difference. This meant they had to switch to trucks which are old and unreliable and they got bogged down in mud, which meant they had to stick to roads where they were easy for the Ukrainians to attack. And those transshipment points where the Russians had to unload all their cargo from trains and reload it onto trucks also became fat juicy targets.

Yet another example where Russia didn't think carefully enough before invading.