Alco S-1 and S-3, Class AS6

The ALCO S-1 and S-3 were 660 horsepower switcher diesel-electric 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. 

The S-1 and S-3 are distinguishable externally from the very similar S-2 and S-4 1,000 horsepower 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.

From http://yardlimit.railfan.net.
S1: narrow radiator, Blunt trucks	S3: narrow radiator, AAR trucks

Alco S-2 and S-4, Class AS10 / AS10m

The ALCO S2 and S4 were 1,000 horsepower diesel electric switchers produced by ALCO and Canadian licensee Montreal Locomotive Works (MLW). The two locomotives differed only in trucks, with the S-2 using ALCO "Blunt" trucks, and the S-4 using AAR type A switcher trucks. Both were powered by ALCO 539 turbocharged, 6-cylinder diesels. The S-2 was built between August 1940 and June 1950, with a total of 1502 completed, while the S-4 was constructed between June 1949 and August 1957 (MLW until 1957) with total sales of 797. Canadian production of the S-4 started more than a year before U S production of the S-4. ALCO did not start building the S-4 until August 1950.

The exterior of the locomotives were styled by an Alco engineer, Ray Patten, using curves in a mild application of Art Deco principles.

The S-2 and S-4 are distinguishable externally from the very similar S-1 and S-3 660 hp switchers in that they have a larger exhaust stack with an oblong base and a larger 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 larger stack is due to turbocharging. The carbody and cab of late S-2s are nearly indistinguishable from those of S-4s. Hence, a truck swap can cause many to misidentify a unit.

From http://yardlimit.railfan.net.
S2: wide radiator, Blunt trucks	S4: wide radiator, AAR trucks

Alco T-6, Class AS10a

The Alco T6 (DL 440) was a diesel-electric locomotive of the switcher type rated at 1,000 horsepower, that rode on two-axle trucks, having a B-B wheel arrangement.

The 'T' stood for 'Transfer', meaning this locomotive was capable of faster transition and higher sustained speeds than the regular 'S' - type yard switcher series.