bajinnova posted:... But I was also curious about the claim that you only have to hold the unload button down until the car starts to operate -- that would make sense if it's using the control rails as a trigger but getting power from the center rail.
For the operating cars I've seen, the instructions say you must activate the car for about 2-seconds to start the self-completing cycle. During the startup 2-seconds, the 4th & 5th operating rails supply power to a DC gearmotor which slides the door over a lever microswitch. Once it passes the switch the power to the gearmotor is then supplied by the center rail. The center rail continues to provide power to the gearmotor until the door returns over the lever microswitch and the cycle is complete.
So it takes more than a momentary "trigger" to start the cycle. In the middle of the video you can see/hear some "false starts" where the trigger was too short and the door did not slide over the microswitch.
As mentioned by others, the sliding-shoe pickups can present clearance issues with certain track systems. In such a case a bell-detector circuit could be used to sense the DC offset voltage on the center rail and enable the gearmotor for ~2 seconds until the track voltage takes over to complete the cycle. DIY DC-offset detector circuits are only $1-2 in parts and have been discussed in many OGR threads though usually for detecting Whistle/Horn. But this does get you operation anywhere on the layout rather than just over a UCS or Operating Track section.
For that matter, for about $5 in parts, I've posted a DIY to use a wireless remote to provide the 2-seconds of startup trigger to activate the operating car anywhere on the layout. The wireless remote has the advantage (over the bell method) in that you can uniquely address a specific operating car rather than all responding to a global bell command which would trigger all operating cars so equipped. As I recall there was muted interest since it requires component-level assembly and soldering. I dug up this photo of the components and a short video. No 4th/5th rail operating shoes required. All power comes from the center rail with only 3 wires patched in to the existing MTH circuitry.
The video again demonstrates the ~2-second trigger needed to move the door over the lever microswitch which starts the self-completing animation.
