If you are using MTH DCS then you need an isolated rail where the engine can be added or programmed to the system. So I use my turntable spurs as multiple isolated rails, since the engine is already sitting there.
When " MTH Program" mode is switched, all tracks and spurs have power cut-off and only the isolated spur is active. Then I program that spur with that engine.
So to quickly add all other engines on the spurs I simply switch the power to that spur via a rotary switch and then set Program mode switch once again.