I think you'll find that people largely wore what they had. Especially crew members who were not passenger-facing. Remember this was before mass-produced clothing, so what few pieces people owned, had to be versatile. The big roads probably had uniform suppliers and more stringent dress codes, but shoestring operations like the WW&F didn't concern themselves with such things.
If you know any thespians, ask them where they source costumes. Try looking in thrift stores. This is one area where close enough is good enough. Especially in the cab, you want something first and foremost that is functional.
Something else I've found: develop your own style. That will help you "feel the part." Many volunteers have developed their own styles within the realm of "generally period-looking" and even though the rivet-counters might balk, people eat it up. Some examples that spring to mind: Bob, Ed, Steve P, Dan, Bryce, JB, and Josh Recave among others. I can identify any of these folks from 500 feet away just by their clothing.
My forum avatar shows the top half of one of my usual costumes: wool cap, Bean flannel, jeans, and steel toes. I also wear a honkin' strap leather belt with a PRR K4 on it. That's definitely not period, but that's what I have. There are many ways I could make it more correct: Better would be suspenders (even moreso buttons). Better would be traditional-cut pants, not modern fit jeans. Better would be welted soles instead of glued soles on my boots. Better would be brass aglets, not polymer. Better would be wire-rim glasses, not plastic ones. My point is you can chase it to oblivion if you want. Or you can pick something close and own it.
