Let me see if I can respond to some of the questions raised by Ken, Mike and Matthew.
There's nothing wrong the TOM siding being a runaround, except that it never was "back in the day." The operation there would be more interesting with a second engine to pull the train into the clear. However, dropping cars into the siding, if the train crews are properly trained, should not be a problem. In fact, passengers might enjoy observing this procedure. The original WW&F did this every day at Albion (and perhaps at Winslow, way back when), as did the Strasburg Rail Road for the first couple of years it operated.
I suggested getting around the train at TOM rather than Alna Center because of the grade at AC and the time it takes to get around there.
Making the TOM siding double-ended wouldn't be that hard. The north end would have to be graded down to meet the main line, but that's what they make dozers for. Park the coaches against a skate with a hand brake on, and they aren't going anywhere while the engine is running around.
And if no. 51 was stationed there to effect the move, it would only run for a couple of minutes each hour, just long enough to couple to the train and pull it into the clear and cut away. The engineer or fireman on the regular train could run the gas engine.
And we could conceivably run two trains on special occasions, with the trains passing one another at Alna Center.