One of the beauties of Marx was it's simplicity. This prototype Mercury is not a shining example which is probably why it wasn't produced.
The engine and tender appear to be permanently coupled with a wire running from the engine to the coupler release on the tender. I guess they were looking for a remote controlled device to uncouple the entire train.

This picture shows the special frame required and the wire to the coupler.

(Courtesy of Kieth Vannatter)