It's probably better to think about what Sisko's division was, because everyone IS on the command track in some way - just not necessarily towards command of a Starship or other facility. LaForge was in the command division for a while, but switched to Engineering where he commands that section.
I also thought that Sisko was in the Engineering division, perhaps on track to become chief engineer of something at some future point. Then Layton recognized real command qualities in him, and convinced him to become his XO aboard the Okinawa. The rest is history - though it does seem a bit of a demotion to go from XO of an Excelsior-class ship to the relatively humble Saratoga afterwards. Was it the next available XO posting after Layton got promoted to Admiral? Was the Okinawa lost or decomissioned? Otherwise why wouldn't Sisko stay as XO there?