I previously posted this "schematic" of an MTH operating track:
And this one for the Lionel version:
There are obviously a wide variety of operating cars that can have different trigger requirements. But for several MTH operating cars I've examined, you activate it by applying power to the two slider shoes. You only need to apply this "trigger" voltage for a couple seconds. This starts an operating cycle such as the doors opening and worker pushing a crate out the door. You can then remove the trigger voltage from the slider shoes and the cycle completes by itself. It does this by drawing power from the normal center/outer rails.
There are some MTH cars (e.g., signalman caboose) which can be triggered by a momentary conventional bell command. If you happen to have one of these cars then you don't have to mess with awkwardly attempting to apply voltage to the slider shoes.