If you are looking for records on Fairmont speeders contact the:

Martin County Historical society
304 E Blue Earth AVE
Fairmont, MN 56031-2865

The records are organized by number so you can't ask for everything from a given railroad.

I would add, there is a fee per engine number requested and you need the engine number that is stamped above the crankshaft on the poppers. I don't know what number was used on the bigger cars with engines purchased from other vendors. Replacement popper engines have no number, so unless the railroad stamped it on the new engine your out of luck as to the history of your car.
Interestedly I have a old popper engine and they have two build cards for it with different customers and slightly different dates.
FYI, they send you photo copies of the cards.
Mike N.