That the country and the war effort would grind to a screeching halt without the railroads. They were obviously necessary in their jobs.

Women might take up welding in the shipyards, but they didn't move into railway jobs. (Our Mothers' War: American Women at Home and at the Front During World War II by Emily Yellin) As it was, it was still an effort to get even enough of the normally excluded workers like women, Negroes, and Indians, into the factories. That men like engineers were usually grey-haired doubly excused them. The jobs were based on experience, long experience.

And they did go into the service. As I said, running railroads took experience, so it was younger railroadmen, and some greying ones, who formed the railroad battalions that got the European railways working to back the Allied progress. History of the 718th Railway Operating Battalion Transportation Corps can show you them in action.