Standards will rise naturally
Even back before WW2, Europe had fairly standardized railways, even though the nations had long histories of war. In peacetime, the railways were very much what tied the continent together, carrying mail and goods as well as passengers. In wartime, armies could use rails to carry supplies and soldiers, and for that, it was also practical to have the same railway gauge. So even on a continent of often hostile nations, it makes sense to have standardized rails, or else goods would have to be offloaded, moved and loaded at every border. In fact, today most of the world uses a global standard gauge of 1,435 millimeters, used in China, most of Europe, North America, most of Australia, Iran, and Turkey.
The Soviet Union had a different gauge of 1,524 millimeters, so the invading German army had to change the tracks as they moved forward - and the Soviets then changed them back as they forced the Germans out. This gauge is still used in most ex-Soviet nations.
My point is that you don't need a centralized authority - public or private - to have standardized systems. It is simply easier to lay tracks that match your neighbours'. If you have ambitions to expand into your neighbours' territories, this is also practical.