After playing PBEM for some time, I have come to the conclusion that rail pools are simply too big. I had no problem transfering entire armies from one to another part of the battlefiled in 15 days. Although rails were used intensivelly in RL history, they were not used at this scale. It was simply not possible. This degenerated the game from holding frontlines to holding 1-2 strongholds from which it can be attacked/counterattacked without penalty at will. Someone holding his units in a front line could not compete with this schwerpunkt strategy. The problem is not the strategy, but the historical transport limitations which made that strategy impossible in 1916.
Let's put some number to my claim. A division s worth only 3 rail pool weight. So, if for example Russia has 600 rail pool, it can move 200 divisions at will any given turn. That is 200×12.000 = 2.400.000 men every turn. You see what I am aiming at, that was simply not possible.
I would like to propose increase in infantry and light artillery weight from 1 to 2. It would help with the issue, and it would not be too drastic change that might overfix the problem.
Supply wagons and plain weight should remain the same.