This is a strategic move: it makes it easier to move weapons within Europe and makes it much harder for Russia should they invade. Ideally you would want to do this all over Europe.
Most of Europe is already on the same track width. I'm not sure whether the loading gauge (allowed size of train to fit under bridges) etc. is also standardized; it wasn't for the UK, which is why we can't have nice things like double decker commuter trains.
Those are not nice things. Double decker trains take longer to load/unload than regular trains for only a small increase in capacity. Single deck trains can make more stops in the same amount of time thus serving more people, or they can take less time in the stops thus getting people where they want to be faster. Time is important to humans, anyone who says slow down to others has no idea how they live or where their needs are. If you want to slow down and smell roses that is fine: go to a park and do so - meanwhile a lot of people need less time on transit so they get more time at home with their kids (or whatever else they do in life)
Larger loading gauges are a good things for a lot of reasons, but the ability to run double decker trains is not one of them.