From this link, I found this:
http://www.slotcarillustrated.com/index.php?categoryid=34&pid=2&p2_articleid=503
It does not look like it has a high wattage variable resistor built into it. I'm guessing that the OP has something using technology similar to this.
Edit
And here is one that even runs on 2 AA batteries so I guess the power circuit is remote: