With the introduction of the computerised Traffic Monitoring System (TMS) in 1979, the class was renumbered. The DX class received four-digit numbers starting with 5, in which the fourth number is a check digit for the number. The locomotives were numbered in order, with DX 2600 renumbered DX5016 (6 being the check digit) and DX 2648 renumbered DX5517. The numbers 500x, 515x, 534x and 549x were not used as these numbers do not generate a valid check digit, and DX 2639 never received a TMS number as it had been withdrawn by then.