Trains are the backbone of every mobile warfare.
Eastfront : supply via trains, armored trains in combat role and security duties. Railroad lines had been the artery of the wehrmacht, cause supply via road ( rollbahn ) wasn´t sufficiant, and sometimes the lorries did use more fuel than they delivered . Partisan actions against railroad lines, stations caused serious problems
Westfront : if there had been a working railraod system during the allied landings, germany had been able to send more and faster supplies via normandy...
Later Pattons rush was slowed due to shortened supply ( not completely solved with the red ball highway ).
Suppy in any form is essential, movement of armored and infantry forces without a working trainsystem very difficult
These are some of my reasons " pro " trains made by GHQ.
Advantages:
You can make " Sets " and sell them
Axis and allied ( even US, cause many Locos had been in use after D-day in europe
Armored and " normal "
Great variety of types
additional accessories like buildings, rails ,cargo loads for flats
figures too
A complex add on for the existing ranges, with great potential
This is what i mean :

and

this too

and

perhaps ?

and

of course

but also " normal "
stuff

and hospital trains

Trains are undisputed essential in modern warfare, and they would be a really wonderfull add on for the existing ranges of " 20th century " onwards...