That would be a middle single-coil rather than a humbucker though, wouldn't it? Assuming it was the white hardtail Strat with a Duncan Allan Holdsworth bridge pickup and DiMarzio SDS-1s.
That is correct, and yes, I was thinking of that particular white hardtail strat, even if I don't know whether he actually played that one in the studio.
You may wonder why I think "Rumbling Train" is double tracked. I read somewhere that he double and triple tracked many leads on the Ozzy albums, and while listening to "Rumbling Train" sometimes I thought "that is the middle pickup" and then a few seconds later "no, it's the bridge pickup", just to reverse it a few seconds later.
Cheers Stephan