CSX Fresh Pond-Oak Point-Selkirk, then Selkirk-Worcester. CSX operates locals east from Croton Yard (?) up the Harlem line then east on the New Haven Line as far as Rings End Lumber in Darien. They operate a local out of Cedar Hill as far west as Bridgeport, but the two locals never meet. In theory they could, but that would require a car routing Fresh Pond-Oak Point-Croton-Bridgeport-Cedar Hill, then hand off to P&W. Obviously the most direct route would be to hitch a ride on a stone train, but apparently the overhead rights agreement between CSX and P&W prohibits that.
Every problem has an answer and every answer is a problem.