You wouldn't say "be on THE track", just "be on track", and you are using it correctly. You probably wouldn't say "You're still on THAT track aren't you?" Instead "You're still on track [for Project ABC], aren't you?"
A similar expression might be helpful to you sometimes. You can say, "I think we're on the right track." This assumes that there are multiple ways of doing a thing, some wrong but at least one right way, and you have found the right track and headed for success.