At the same time, that "last mile" between your railroads and your final destination can in and of itself be a pretty major logistical impediment. During WWI Germany was able to mobilize far more quickly than its opponents thanks to its railroads, but to traverse the distance from its own borders through Belgium to hit Paris was more than their logistics was capable of supporting since they were beyond the domestic rail network, something the German general staff knew outright. They ignored those problems and launched the offensive as planned anyway, resulting in several years of trench warfare. And apparently not having learned their lesson, they did the same thing in WWII with Barbarossa against Russia. Belgium of course kind of got squashed like a bug both times though, so with enough firepower, it can be done. If you can move quickly enough and your enemy is weak enough. If not, the death toll skyrockets very, very quickly.