ALCO S-1 and S-3

The ALCO S-1 and S-3 were 660 hp diesel-electric switcher locomotives produced by ALCO and their Canadian subsidiary Montreal Locomotive Works (MLW). The two locomotives differed only in trucks, with the S-1 using ALCO's own Blunt trucks, and the S-3 using AAR type A switcher trucks. The S-1 was built between April 1940 and June 1950, with a total of 543 completed, while the S-3 was constructed between February 1950 and November 1953 (MLW until 1957) with total sales of 300. A modified version, the S-10, was built by MLW only; 13 were built between January and June 1958.

Identification
The S-1 and S-3 are distinguishable externally from the very similar S-2 and S-4 1000 hp switchers in that they have a smaller exhaust stack with a round base and a smaller radiator shutter area on the nose sides. The S-1/S-3 radiator shutter area is taller than it is wide, while the S-2/S-4 radiator area is wider. The smaller stack is due to the lack of turbocharging.

The S-10 is not externally distinguishable from later Canadian-built S-3 locomotives; it differed mostly in electrical equipment.

Original owners
The S-1 and S-3 models were sold to an extensive list of railroads and industrial operators, as detailed below. Major owners of the S-1 included the New York Central Railroad (NYC), with 71 locomotives; the New Haven with 65 locomotives; the L&N with 45 locomotives; the C&NW, with 29 locomotives; and the Pennsylvania Railroad (PRR) with 27 locomotives. Major customers for the S-3 included the CP, with 101; the CN, with 49; the NYC, with 43 locomotives; the B&M, with 16; and the PRR, with 13. The MLW S-10 was sold only to the CP.

The totals below include export orders and MLW-built locomotives.

S-1
ALCO constructed approximately 535 S-1s for the US market between 1940 and 1950.

S-3
ALCO and the Montreal Locomotive Works constructed approximately 300 S-3s for the North American market between 1950 and 1957.

S-10
MLW constructed 13 S-10s in 1958, all for the Canadian Pacific Railway, numbered 6601–6613. These units were essentially similar to late-built S3s, though with minor updates to the electrical gear.

S-11
In 1959, MLW built a final order of 660 horsepower switchers for the Canadian Pacific, as model S-11, numbered 6614–6623. The internal machinery of these units was essentially the same as that of the S-10, but the car body was radically redesigned, with the radiator on the front end of the hood instead of on the sides.

Preservation
Numerous S-1 and S-3 locomotives remain in use, and several are preserved:
 * Ex-U.S. Army S-1 No. 7372 is at the Western Pacific Railroad Museum and painted in Western Pacific colors.
 * Ex-Canadian Pacific MLW S-3 No. 6568 is at the Saskatchewan Railway Museum.
 * Two Ex-Steel Company of Wales S-1s are preserved and are being restored to working order at the Nene Valley Railway in England.
 * Ex-Erie Railroad S-1 No. 307 is preserved at Riverside Park, Manhattan and painted in New York Central colors.