If the CPs control Marseille and Constantinople and a chain of tts connecting them why not move Turks to Marseille? The Allies can make the same move in reverse by sea. It’s up to the Allies to block such moves. Remember you can never rail units into combat, only to reinforce contested tts.
The proposal is to counter the Allies domination of the seas, which they usually get after 2 or 3 turns, and means they can move troops virtually immediately to any front.
Yes, the actual combat in WWI was slow and static, but as I mentioned this came about precisely because they could move troops so quickly from the depots to the front lines by rail.
Requiring German infantry to “march” one tt at a time from Berlin to the front lines fatally cripples the CPs in OOB rules. The Allies can easily see where they’re heading and send troops there by sea. In my view rail movement is essential to balance this game.