There's been some track work including bridge reconstruction recently, so it's likely the loco ran out to the east end, and wasn't sent back west before the trackwork started, thus the locomotive was "stuck" until the work was complete.

Also, if the engine ran out to Mill City and didn't have any loads to haul back or wasn't needed in Lebanon - it may have been cheaper to let the locomotive sit there and taxi the crew back to Lebanon instead of paying for fuel for a locomotive to make a 35 mile light move. Trains only make money when there are loaded cars behind the locomotive, and why pay a crew for several hours of "work" and fuel to do essentially nothing, when a crew hauler can be sent after them and get back into Lebanon for just a half hour time, and a couple of gallons of gasoline?

Just speculation...