Hi All – Baltimore, Maryland, USA has a museum called the “B&O Railroad Museum”. B&O stands for Baltimore and Ohio.
You can find vintage cards all over eBay and elsewhere, but currently none on their website at https://www.borail.org/
Hi think I just talked one of their employees (Eileen) into offering postcards again on their website as they apparently do still have them in the walk-in museum shop. She thought there might not be enough interest in postcards to offer them online.
If you get a chance, please NICELY inquire if they plan to start selling them online. They are tentatively planning to add them but I’m sure a bit of encouragement could not hurt! 
This is their contact form: Contact Us | B&O Railroad Museum
I already sent her:
Thank you!
Not a giant opportunity, but postcards are a growing hobby. Postcrossing claims 802,819 registered members in 208 different countries ( https://www.postcrossing.com/stats/users ).
Try entering this phrase in Google for an idea of how much train discussion is on postcrossing.com. Google claims about 2300 matches: "site:postcrossing.com trains"
You may pick-up some mail order business from: 1) train buffs, and 2) people trying to represent Baltimore and Ohio when asked to send train postcards.
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Thanks,
Michael