If you can get some steering gear, that's probably half the battle. A decent sized rare earth magnet, maybe on a couple of tiny wheels rather than a skid, and some steel rod embedded in the roadway. Ebay mike makes a good point about the drive - I reckon you need to keep the speed down, and the track radius generous. What you are really trying to avoid is driving the magnet off the track, as far as I can tell. The only other advice I've been given is to get the magnet as far in front of the steering as you reasonably can.
To be fair, I considered this for a couple of Wilseco tractors, but they are way too heavy so I'll cop out and go for a single rail or a groove. Thats it I ever get round to it.