Its a coupler around one meter long. One ends fits straight into the auto coupler, via a manual hook, and the other end fits straight into the housing on a trailer car. This end is held in place, with a large solid pin.
Thus any trailer car car be moved by a four car unit around the depot. Hardly used these days,a part from rat duties.
Saw the RAT in Uxbridge Sidings ~ 13:30 this afternoon. At first I thought it was some advertising livery, then I looked again and it was the RAT.
I think it would look a bit neater (and may do better for deterring passengers) if they vinyled Rail Adhesion on the other cars aswell. I know that it generally passes through stations rather than stopping, but inevitably it will stop, and one day some idiot is going to try and get on and hurt themselves...