Sorry for the delayed response, couldn't find the thread..doh!!
Thanks everyone for the replies, seeing the coupling up-close explains it well (thanks for the links jo). Chris, I like your solution, and I might have a go at a "rescued" power car now for the TMD I'm building. Would be an excuse to buy / use a dummy power car and have it "dragged" into the depot for some urgent repairs!!
Thanks once again everyone.
Mike