I didn't know what this was when I bought it, and neither did the stallholder who sold it to me.
However, I was curious enough to splash out 8 quid to have a look. The Gaugemaster badge at the bottom told me I was unlikely to be handing over my money for rubbish.
The blue/green plastic tray holds two long grey sponges with strips of clear plastic over the top. I guessed it was something to do with running locos and with a little fiddling, a OO model can sit on the sponges with the clear strips holding it in the middle.
Feed some power in and with a little initial prodding, the loco moves.
What witchcraft is this? Electrically conductive sponge? How can this be?
Not only does the sponge move electricity, if you hold the loco in place, it polishes the wheels too. Very clever!
It seems I bought a GM50 Locomotive Wheel Cleaner. Price £45.
Or £37 more than I paid, showing that even on the Sunday afternoon at a show (remember them?) there are still bargains to be had if you look hard enough. I've added this to my loco maintainence kit where I'm sure it's going to be very handy.