The roundhouse is a remnant from the steam locomotive days. It was a large brick building with a turntable in the center, surrounded by a dozen stalls where locomotives were repaired and stored at night.The noun, hostler, originally referred to the stableman who made sure that flesh-and-blood horses were not put away wet after being ridden hard. The iron horse required similar care and feeding.
The locomotives were driven onto the turntable, which would rotate, allowing the trains to pull into a stall. The building had a coaling tower and water tank. At night, workers called "hostlers" fed the boilers to keep the steam engines running.
It's unfortunate that relatively few turntables have made it onto preservation railroads. There's drama in bringing a steam locomotive out of its stall, balancing it on the turntable, then turning it to move to the ready track and thence to station or yard.
The archaeological report Massachusetts will receive promises to be of use here, as a model Salem engine terminal is part of the model railroad still a-building down cellar.