A battle front so static, you could build a rail system to supply it. What a dismal end of track.
You miss a couple of points about the system of field railways (both France and Germany had a well-developed plan for using these).
First is that it's relatively easy to build new track where you need it (including after a putative 'breakthrough' of the trench line). Notice in the first scenes how the 'ties' are round and there is little formal ballasting. The French, I believe, had whole sections of track that were laid down ahead of an advancing train and then picked up after it had passed for 're-use'
Remember this is in the era of hard-rubber-tired trucks that wouldn't last a moment before bogging down, and that would require carefully decked bridges. And limited capacity per truck and driver -- this was in an era when truck-driving was something of a special skill.
Look at the film toward the end and you see one major 'advantage' in trench warfare. You may recall from your WW1 battle history that the idea of a breakthrough involved mass assaults by troops as well as artillery, etc. There is probably no better way to move 'rested' troops, with all their gear and weapons, between points 'behind the lines' than the sort of train you see, with convenient 'poles' to hang onto as the only superstructure. No particular warning to the enemy, even with contemporary air spotting, where the train is going to stop, either.
In other words, capable and flexible. NOT particularly dismal or retarded-development.