Just spent a few happy hours rounding up all locos and arranging them neatly around the turntable. After putting each loco in position, I shut down its engine. At the end, I expected silence, but always there was the sound of engines running. It seemed that when viewing a loco from outside, engine was running and smoke coming from exhaust. If I boarded that loco, internally, engine was not running, as I couldn't move the loco and I could discharge the main air reservoir without the compressor recharging it. It seems that external view and external sound of a loco does not take into account whether engine has been shutdown. This applies to GP38 and AC4000 loco types.