One day, we went to the Baltimore and Ohio Railroad Museum in Baltimore. We got lucky that day and saw the staff turning some engines on the turntable. One of them is No. 25, the William Mason, a classic American 4-4-0 steam engine from the middle of the 19th century. After its retirement from the B&O and subsequent restoration, it has starred in such movies as The Great Locomotive Chase, Tuck Everlasting, and Wild Wild West.
That roundhouse was built as a coach storage roundhouse. The turntable pit didn't need to be well drained and they could afford to build a floor over it that prevented workers from falling into the pit. When this facility became a museum, it was a perfect fit.