My opinion will likely be an unpopular one, but I think the more specialized items like rail guns, armored trains, recon units, anti-tank (and the wunderwaffen missiles too) should have simply been omitted given the map scale of this game. They might have made sense in the other SC titles where you have more game tiles to work with. But at this world-wide scale and with the one unit only stacking limit, you sometimes fight to find room to properly position all the corps, armies and air fleets that do most of the real work. England gets so packed with various bomber types and escorts and sooner or later with infantry and paratroops for the Normandy invasion that it becomes difficult to find room to accommodate everyone. Sometimes less is more.
But given that these exotic unit types probably have their devoted fans after featuring in previous titles that likely would have been a tough proposition for the game's developers. "Why did you cut my favorite weapon?....Bwahhh!"
My two bits.
Oh, I agree. All of those units should have been omitted - and precisely because of the scale of the game, as you say. For instance, the Soviet rail guns seem to have been 16 inch caliber; and an Iowa class battleship alone had 9 16 inch guns. But I am assuming those units are here to stay, and the devs won't want to completely delete them.