The speed up of troops a poster previosuly mentioned should take the form of replacing the units movement value with a new one while in provinces with railroads, rather than a percentage modifier.
Using the example of two units, infrantry with a base speed of 4 and hussars with a base speed of 7.
A level one railroad should give a base speed of, say, 14 to all units using it rather than a percentage of modifier which would result in trains carrying horses nonsensically travelling twice as fast.
When moving into a province with enemy units, even if both provinces have railways and/or it is one of your controlled provinces, the units base speed should be used rather than the railway speed as the units move from the railhead to the battlefield and so that the speed advantage of cavalry in tactical situations will still be advantagous.
Secondly, divide railroad level into two parts, railroad technological level and railroad coverage. Railroads should also be their max technological level when built (and perhaps be automatically upgraded when there is the cash or capitalists to do so) but their coverage would start at level one and be upgraded by government and capitalists normally. Higher coverage would be needed to support greater numbers of troops using them and more heavily developed industrial area.
For example, a level 5, coverage 1 railroad in a province would have the same effect on a 10 000 man army and/or low level factories in a province as a level 5, coverage 5 railroad, but would give a smaller percentage effect (and the same absolute effect) to a 200 000 man army marching through or high level factories in a province compared to a level 5, coverage 5 railroad.
I would suggest Paradox add a rudimentry supply system into Victoria 2 requiring rolling stock to be built and maintained in order to receive the rail unit movement speed rather than their typical speed. For example, each 10 000 men moving in rail provinces would require one theoretical unit of rolling stock in order to receive that. The groudn work already exists in HOI3.
If that is too much to ask for, have each level of railroad coverage in a province support giving the railroad movement speed to, say, 50 000 men at once.