Part of our goal for Lego World 2014 are two independent running trains, that couple to the wagon of the chosen color. Therefor, one of the requirement elements is an automatic wagon decouple mechanism.
Since the trains are already controlled by the NXT brick plus HiTechnic IRLink, the simpliest solution would be to add the decoupling mechanism via a NXT motor to this wagon. But these motors are not easy to build-in if you have little space left on your train.
The proof of concept that I made, is now based on the EV3 M-motor and two linear actuators. I used the EV3 M-motor because of its rectangular shape and its smaller size. Here is a sketch in LDD.
The orange pieces pushes the wagon about 3 cm from the train, which is barely sufficient to overcome the magnetic force of the train couplers. The clutch gear (element 60c01) is added to ensure that the internal gear inside the actuators is not damaged.
A video of the automatic train decouple mechanism can be found at our Youtube account.