Nath123, you can understand logistics, aspects of warmaking, etc.
But you can do nothing with one fail-point - your capital.
Supplies system seems broken due to concept "capital only produce supplies".
Try USSR for example. You have about 350 divisions at start.
You have 3000+ supplies demanded every day.
And the throughput of your capital is around 600.
You can do nothing about it, it's not infrastructure limiting you, but the concept that supplies appears only in capital and no other place.
The war preparations means that you'll establish factories all over the country and make some depots which seems reasonable to have when your march begins.
Supply system doesn't count way of supplies from factories to capital.
Instead you assign X IC to supply production and have all of them appear in capital. In current implementation you have no chance to deliver more to your units while you are overstocked with supplies in capital.
In "real" situation you can't imagine to have developped industry (part of which would produce supplies) in Far East, then deliver all supplies from there to Moskva (8000 km) and after that try to supply all divisions in Far East from Moskva over the same 8000 km back.
If you understand logistics, of course.