I understand by it that railroad schedules (that we mortals call railway timetables), and the documents of railroad companies, in the States, use ordinary type for am and bold for pm so you know which is which without having to have am and pm printed. (Where, in continental European countries, you have the 24-hour clock so no ambiguity.) "Railroading" being "the practice of railroad companies, workers, people".

I am probably completely wrong.