I've made this tiny decoupler rail car from spare parts I had at home. I am not sure but I think it could be the most inexpensive and simple decoupler which uses the pushing car system. It is also very easy to build and it has enough force to assure a pleasant play experience.
More pictures could be seen here: https://www.flickr.c...157663113059113
LEGO small decoupler 4 by Severus A, on Flickr
LEGO small decoupler 2 by Severus A, on Flickr
LEGO small decoupler 11 by Severus A, on Flickr
LEGO small decoupler 10 by Severus A, on Flickr