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Without knowing what the track or the roller look like it is hard to say. Cneterline mate to the face of the track (offset the radius of the roller) or tangent between the surfaces is the best I can tell you without seeing it.
You could use either Tangent constraint or Transition constraint. Tangent: Pick the roller outer face (the one contacts the track) as first selection and pick the innert face on the track (the one contacts the roller) as second selection. Transition: the same as above.
Hope this helps a bit.