it's done with memory wire linked to a decoder in the wagon assuming your talking Kadee's. I've only seen this done with American H/O - they have nice long reefers.
The seep uncoupler is a ponderous contraption demanding almost pin point accuracy to work well. Rather investigate Kadee couplers, they work very well and uncoupling, is pretty much guaranteed rather than left to chance.
Krois and Roco (which I think is is based on Krois) are the one I know about. Zimo decoders provide support for these deocoders, see this extract from the manual :-
QUOTE In conjunction with the electric uncoupling (system Krois), it is possible to define the locomotive de-coder so that it automatically pulls forward a specified distance away from the train while the cou-pler is disengaged.
There is a lot more about how to control this and other features of the Krois or Roco system in the Manual for the MX620, MX63 and MX64 family of decoders. I am not quite sure if this is the kind of system you are looking for, so I'll stop here.
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