In the case of two books I relied upon various train records. For the Ross biography, I had access to Ross' own notes and travel logs via telegrams as well as ticket stubs and letters mentioning train times. I also went back to look at train routes.
With the Sebonack book, we used Long Island Rail Road records, maps, timetables, posters.
It's amazing how having that stuff in your hands helps you appreciate the conditions under which they worked.