The HH series were the first diesel shunting locomotives that ALCO mass-produced in large numbers. They were also among the first land vehicles with a diesel-electric transmission. “HH” stood for “High Hood” while the number stood for the engine power in hp. The first HH600 demonstrator was sold to New Haven after testing in 1931. In 1934, industrial designer Otto Kuhler revised the shape of the locomotive body, giving it more rounded corners.

HH600 No. 913 of the New Haven on a 1938 ALCO advertising brochure

ALCO

The first variant had a McIntosh & Seymour 531 with six cylinders. It wasn't until 1937 that the HH900 followed with a turbocharged variant of this engine. In 1938 the HH660 followed with the slightly more powerful naturally aspirated engine of the 538 series and from 1939 finally the HH1000 with the turbocharged variant of the engine of the 538 series. A total of 177 locomotives were built, of which the HH600 was the most successful with 79 units due to its long production period. The individual railway companies operated a maximum of twelve of each model.

To replace inefficient steam switchers, ALCO introduced the S series as a successor to the HH series in 1940. The new engine in these locomotives was now marketed under the ALCO brand, while the earlier switchers had engines named after the name of ALCO's subsidiary McIntosh & Seymour. All models had two trucks with two axles each, electric power transmission and a cab at one end. In comparison with the HH series the hoods were now lower, providing a better view from the cab. A considerable number operated well into the 21st century.

The S-1 had a naturally aspirated engine delivering 660 hp. In 1950, it was replaced by the S-3 that had the same power, but new trucks. In total, 543 S-1 and 300 S-3 had been built by November 1953. As a more powerful variant, the S-2 was also introduced in 1940 with a turbocharged engine delivering 1,000 hp. As early as in 1949, this one was replaced by the S-4 with new trucks. By 1957, production had totaled 1,502 S-2 and 797 S-4.

In 1954, ALCO started their first try to introduce their new 251 engine into the switchers. So they offered the S-5 with this engine, but since it only delivered 800 hp, only seven were built. But the S-6 with a 900 hp variant of this engine that was offered starting in May 1955 was a success. By December 1960, 126 had been built. The Oliver Iron Mining Company received two cow-calf pairs designated SB-8/SSB-9. These consisted of one regular S-6 and a second one without a cab, which were permanently coupled together.

The prototype of the DT-6-6-2000 was commissioned by the Elgin, Joliet and Eastern Railway, which ultimately used 27 of the 45 locomotives of this type. These were transfer switchers for handing over freight trains between marshalling yards. A central cab lay between two engines. Two naturally aspirated eight-cylinders were used in the prototype, which were replaced in series production by six-cylinders with turbochargers. Many locomotives were later modernized with new engines from Baldwin or EMD, each with 1,200 hp. 

In 1935 EMC manufactured a total of three prototypes of a heavy switcher, which were equipped with the Winton engine, of which the E-series had two. Two of these switchers had a 600 hp in-line 8-cylinder engine, while the third had a 900 hp V12. All had the same frame length, but with the more powerful version, the body was longer. A special feature of the hoods over the machinery was that the ceiling tapered to the driver's cab to improve visibility.

The production locomotives received an S for “six hundred” in the version with 600 hp, while the version with 900 hp received an N for “nine hundred”. There were also two different types of frames, which can be distinguished by the second letter in the type designation. The first version was cast by General Steel Castings and therefore has a C for “cast”, while the other was welded by EMC themselves and therefore has a W for “welded”.

The SC, SW, NC and NW types initially resulted from the combinations of engine and frame. Other versions of the more powerful variant were built, which differed mainly in terms of the electrics and were designated NC1, NC2, NW1 and NW1A. Two more copies were made from the NW4, which were lengthened and had an additional steam generator. These were used by the Missouri Pacific for switching passenger cars. A total of 175 series locomotives were produced up to 1939.

A single one was only given the type designation T and basically consisted of two power trains from the N series, which stood on four two-axle bogies with a driver's cab in the middle. With an output of 1,800 hp, it was intended to transfer freight trains between nearby marshalling yards and was not built in series because the long frame settled over time.

In 1949, EMD introduced the SW7 switcher as the successor of the NW2. It still had the twelve-cylinder 567A engine, but now with 1,200 instead of 1,000 hp. Like its predecessor, it had two exhaust stacks on the hood. 489 were built until January 1951. 15 more were built as permanently coupled cow-calf pairs where only one locomotive had a cab.

In 1950, the SW8 was introduced as a less powerful variant with an eight-cylinder that delivered 800 hp. It could be distinguished by the single exhaust stack. 309 were built by EMD in the USA and 65 for the Canadian market were built by GMD in Canada. 41 of the locomotives built in the USA were delivered to the Army for use in the Korean War and many stayed in South Korea after the war. The cow-calf variant of the SW8 was called TR6 and twelve were built.

Two months before the completion of the last SW7, the first SW9 was built. As its direct successor, it still had 1,200 hp, but the engine was now of the more modern 567B variant. When production ended in December 1953, 786 had been produced by EMD and 29 by GMD. The TR5 cow-calf variant numbered ten pairs with two additional boosters built. Soon, this generation of switchers was replaced by the SW900 and SW1200.

From 1944, Fairbanks-Morse offered the H-10-44 as a yard switcher with a two-stroke, six-cylinder, opposed-piston engine. The exterior was designed by Raymond Loewy, which could be recognized by a slanted nose, sloping shapes and a protruding cab roof. After many had been later set to 1,200 hp, the H-12-44 with the same power was offered from 1950. From 1952 the exterior was simplified to save on production costs. The H-12-44TS was a road switcher with a short hood in front of the cab. This variant was only built three times for the ATSF.

In the 1970s, a heavy shunting locomotive was needed to assemble heavy passenger trains, since the existing WDS-4 were no longer up to the job. The new locomotive was given the designation WDS-6, which stood for “Wide Gauge, Diesel, Shunting, 6^th Generation”. It had the same powerpack as the meter-gauge YDM-4, i.e. the six-cylinder ALCO 251-D with an output of 1,400 hp. Production started in 1975 at the Banaras Locomotive Works in Varanasi. Later, also in Parel, locomotives were assembled from components that had been manufactured in the BLW. Of the 411 WDS-6s built, many were later sold to industry or public sector undertakings.