That’s a contributing factor. Their land mass, topography and general population density benefitted it highly as well, but the need to reconstruct their cities provided two problems that needed solving - fix the previous infrastructure and get the workers from outside the demolished zones into them. Trains existed for a lot of things already so it made sense to keep pushing them into the city centers. It was an inexpensive and reliable solution.

They had it, but WW2 did so much damage (WW1 contributed to this too) and the need to move workers around cheaply and reliably to rebuild gave them an opportunity to improve public transit and make it the primary focus for transportation infrastructure. Build the transit route into the communities or commercial areas, then rebuild those zones around it. By the 70’s it was fully modernized and a primary focus from governments. Without that destruction it may have just been city centers like North America does.

Perfect opportunity to build from scratch.