Alco "S-6" Locomotives

The Alco S6 was the sixth model of switcher series the company produced and debuted in 1955 and built until 1960. The S6, up to that time, would prove to be the American Locomotive Company's (Alco) least successful switcher design with fewer than 130 purchased and none were ultimately built by the Montreal Locomotive Works. The model was based from Alco's custom S5 locomotive built exclusively for the Boston & Maine Railroad, which had asked for a switcher with a bit less horsepower. Similarly, the S6 offered less power than the S4 as Alco looked to capture the market for such but ultimately would not find a lot of interest. 


Still, a handful of railroads and industries did purchase the S6 including several foreign lines (the Southern Pacific would wind up with 55% of the total number built). Today, at least two S6s are known to exist; Southern Pacific #1034 (at the National Railroad Museum) and #1051 (in operation at the Niles Canyon Railway). Both units still carry their original number and livery.

Southern Pacific S6 #1085 appears to be tied down with its caboose at Newhall Yard in San Jose, California during August, 1964. Drew Jacksich photo.

The Alco S6 (designed by the company as its DL430) was quite similar to all of its previous siblings, and as aforementioned was the successor to Alco's S5 design. The S5 was the first switcher to employ Alco's very own prime mover, the model 251A, which was capable of producing 800 horsepower for the design. Interestingly, the builder decided not to use its troublesome model 244 prime mover in any of its switchers, which was unsuccessful in its main line freight locomotives like the PA and FA but had worked quite well in its early Road Switcher (RS) line (the RS1, RS2, and RS3 in particular). In any event, whereas the S5 used Alco's 251A diesel engine the S6 used the updated 251B. Additionally, the S6 came equipped with 900 horsepower whereas the S5 was only rated at 800. Also, just as with the S3 and S4, the S5 and S6 were equipped with updated AAR type A trucks and not the Blunt trucks used on the early models.

As you can see, both designs were quite similar especially the external carbody. Both units were 45 feet, 5 inches in length and weighed 115 tons. The most noticeable differences could be internally (aside from the slightly different prime movers). The S5 offered dynamic braking and the S6 did not (it was the only model that did not offer this feature). However, the S6 featured a higher starting tractive effort, 61,500 pounds compared to the S5's 57,500 pounds. And, as was the case with almost all Alco models of any type, both used traction motors and air components from General Electric and Westinghouse.

South Buffalo Railway S-6's #43 and #44 roll into C Yard on July 29, 1984 with S-2's #103 and #106 in the distance. Note the Baltimore & Ohio unit to the far left. Doug Kroll photo.

It should also be mentioned that Alco also offered a cow-calf version of the S6, the SSB-9A/SSB-9B. This design was built exclusively for Oliver Iron Mining of northern Minnesota. The operation moved and shuffled ore jimmies for the Duluth, Missabe & Iron Range Railway that served the mines north of Duluth at locations like Ely, Mt. Iron, and Coleraine. Oliver Iron fleeted a large collection of switchers and was interested in as much horsepower and tractive effort as possible to move the heavy ore (which typically weighed about twice as much as coal). Essentially, the SSB-9 cow-calf was setup was simply an S6 with a cabless S6 semi-permanently attached. In total, Oliver Mining purchased two sets of this locomotive; #1217A-1217B and #1218A-1218B.  For more information about the S6 please click here.  

One of Boston & Maine's rare S5's, #1282, is seen here dead outside the shops at North Billerica, Massachusetts on June 20, 1978. The unit began its career as #865. Doug Kroll photo.

The Alco S6 sold modestly at just 126 units although they could be found coast to coast among a handful of Class Is like the Southern Pacific (which bought 70) and Western Maryland as well as industries like ARMCO Steel, Republic Steel, and Semet-Solvay. Additionally, Mexican lines including the Chihuahua Pacifico, Cia Fundidora de Hierro y Acero, Ferrocaril del Pacifico, National de Mexico, Secretarias Communicaciones de Obras Publicas, and Unidos de Yucatan purchased 33 S6s between them.  

Alco S5 Production Roster

Owner Road Number(s) Quantity Date Built
Alco (Demo)90911954
Boston & Maine860-86561954

Alco S6 Production Roster

Owner Road Number(s) Quantity Date Built
Armco Steel Corporation120211956
Belt Railway Of Chicago42011957
B. Perini & Sons, Inc.10111956
Columbia Geneva Steel111957
Northern Pacific75011955
Ohio River Company5611956
Republic Steel Corporation153, 317-31831957-1959
Semet-Solvay Company11011956
Sheffield Steel15-1621957
South Buffalo Railway40-4561955-1956
Southern Pacific1033-1090, 4634-4645701955-1956
Tennessee Copper Company10711957
U.S. Pipe & Foundry Company3811956
Western Maryland151-15221956

Winchester & Western S6 #80 leads an RS3 as the two Alcos perform switching work at the small yard and shops in Gore, Virginia during the fall of 1989. The switcher began service as Southern Pacific #4644 in December of 1955. Wade Massie photo.

For a comprehensive look at the American Locomotive Company and all of the motive power types it built from steam, diesel, to electrics consider the book Alco Locomotives by Brian Solomon. Covering more than 175 pages Mr. Solomon's book details the history of Alco from its esteemed 4-6-4 Hudsons and 4-6-6-4 Challengers to vaunted RS and PA series diesel locomotives. If you have any interest in Alco this book is a must have!  Also consider Mike Schafer's Vintage Diesel Locomotives which looks at virtually all of the classic builders and models from Alco PAs to early EMD Geeps.   To read more about other Alco switchers please visit the Diesel Locomotives section of the site, which can be reached from the top of this page.

  1. Home
  2.  ›
  3. Diesel Locomotives
  4.  ›
  5. S-6

Header Photo: Drew Jacksich

Researching Rights-Of-Way

A popular pastime for many is studying and/or exploring abandoned rights-of-way.  Today, there are tens of thousands of miles scattered throughout the country.  Many were pulled up in the 1970's and 1980's although others were removed long before that.  If you are researching active or abandoned corridors you might want to check out the United States Geological Survey's (USGS) Historical Topographic Map Explorer.  It is an excellent resource with thousands of historic maps on file throughout the country.  Just type in a town or city and click on the timeline of maps at the bottom of the page!

Studying Diesels

You will be hard pressed at finding a better online resource regarding diesel locomotives than Craig Rutherford's  The website contains everything from historic (fallen flags) to contemporary (Class I's, regionals, short lines, and even some museums/tourist lines) rosters, locomotive production information, technical data, all notable models cataloged by the five major builders (American Locomotive, Electro-Motive, General Electric, Fairbanks-Morse, and Baldwin), and much more.  A highly recommended database!

Electro-Motive Database

In 1998 a gentleman by the name of Andre Kristopans put together a web page highlighting virtually every unit every out-shopped by General Motors' Electro-Motive Division.  Alas, in 2013 the site closed by thankfully Don Strack rescued the data and transferred it over to his site (another fine resource).  If you are researching anything EMD related please visit this page first.  The information includes original numbers, serials, and order numbers.