Sloreck has it right. My mental image is more of a conveyor belt than a conventional long train arriving from time to time (I was waiting in Canada last month for 1 1/2km of train to go through a level crossing at @20kph). If you can send off a 50 ton load on small trucks every 5 minutes at, say 15mph they will get to the other end at the same rate four days later. Yes I am aware of the need to keep them fuelled and the mind bending complexity of the yards necessary to load and unload them. The latter can be a assorted stations along the initial line slotting where gaps in traffic permit instead of one giant 2D ball of wool. A two line system with a return and outgoing track that permits coaling/fuel trainloads to be deliverd to refuelling points along the line and then return on the other line.
Of course it would be far more complex than a front line railway that just picks up supplies and shuttles back and forth to one loading point and one unloading point just a few kilometres apart and a proper narrow gauge or mainline gauge railway would be far superior but IOTL these were not sufficient and new ones could take months, more like years, to build new ones. My concept is to sacrifice size and efficiency for speed and ease of construction. with small engines and trucks on a narrow gauge you are freed of many of the constraints of large lines and can meander about to access lesser gradients and barrier crossing points. One could pre survey routes clandestinely and stockpile rail track sets, bridges in kit form etc.
Not an easy or sexy solution but one appropriate to the period and ideal for the germans in particular. Of course this would alter the minor strategy of a conflict as the se lines would be items to be taken or defended or needing delays to allow removal or building but it is one way around the period logistical shortfalls.