All RS-18 units were equipped with a small, 1000 Imperial gallon, fuel tank but these came in two different styles. 3615 to 3745 had a longitudinal tank parallel to and well inside the locomotive frame. The air tanks were also mounted longitudinally. Units 3830 to 3893 and 3100 to 3129 had a transverse tank mounted near the cab end of the locomotive with the air reservoirs mounted transversely ahead of this tank. All units were equipped to run long hood forward. 3671-3745, 3830-93 and 3100-29 originally had lightweight MLW trucks but these were later changed to AAR Type-B trucks on almost all units. Dynamic brakes were fitted to 3830-3893 and 3100-3129 (all in the short hood) and can be identified by the additional louvers on the left-hand side of the short hood and on the roof (though the roof fittings seem to be smaller on some units).
The pilots on the front and rear of the 3100-3129 locomotives have a raised central section to accommodate the rigid steam line pipe, which is just above rail height.
As built, there were three variations of louver and filter arrangements on the RS-18 carbody in addition to the variations due to the presence of dynamic brakes in the short hood:
- 1957-8 style on 3615-3745 — five separate square filters in a horizontal line between the intercooler, which was covered with a metal grille, and the cab. See the photo of 3672 at Brockville on 10 August 1960.
- 1958 style on 3100-3129 and 3830-3849 — the intercooler was covered with a metal grille and there was a three unit filter near the cab. See the photo of 3840 at Kingston on 1 April 1961.
- 1959-60 style on 3850-3893 — the intercooler was covered with chicken wire and there was a three-unit filter near the cab. See the photo of 3865 at London on 4 July 1964.
About 1963 Canadian National began to retrofit the RS10/RS10S and RS18 units-with a new layout of the filters — there were three square carbody filters in a horizontal line near the intercooler and a three unit filter near the cab. The covering over the intercooler was probably unchanged — those units that had a chicken wire covering seem to have retained it. 3615-3745 were modified about 1963-1964 and the other units except 3857 (later RSC-14 1760) were modified about 1971-1972.
The carbody air-filters were originally made of a black material but in the 1970s white filters with diagonal, silver stripes were introduced (see photo of 3643) and in the 1980s light blue filters were used (see photo of 3616).
Originally RS-18s 3615-3745 and 3830-3893 had traction motors with 74:18 gearing and a maximum speed of 65 mph. Some, possibly all units later received traction motors with 65:18 gearing and were allowed to travel at a maximum of 75 mph. The CN Mechanical Department Diagram sheets (mod 489) show all RS-18s changed to 75-65:18 with Issue B (c.1960-61) and those done were marked 75-65/18 on the cab side above the number. Later these markings appear to have been dropped, at least on some units.
Some of the 3830-3893 series were retrucked in 1975 and 1976 with six axle trucks to create RS-18ms or RSC-14s (see later section).
The last RS18 was retired in 1993 except for those converted to RSC14s.
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