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Alco "RSD-12" Locomotives

Last revised: May 18, 2023

By: Adam Burns

The RSD12 was one of the final six-axle designs produced in the Road Switcher (RS) series. The locomotive sold relatively well for the American Locomotive Company (Alco) considering most six-axle models were lucky to see more than 50 sales.

The RSD12 was essentially an extension of the RS11, which was Alco's attempt to compete with EMD's four-axle GP9.  Unfortunately, once again the builder was late to the game as EMD's SD9 (the six axle version of its GP9) had already been in production for two years. 

As such, despite railroads continued reluctance to purchase six-axle locomotives in large numbers EMD sold three times the SD9s in comparison to Alco's RSD12. Today, there are at least three preserved although it is unknown whether any are currently operational.


6091208317247126316858920697.jpgUtah Railway RSD-12 #600 (ex-Chesapeake & Ohio #6707) at Martin, Utah, circa 1977. American-Rails.com collection.


With the woefully unsuccessful RSD7, which attempted to compete against Fairbanks Morse's H24-66 "Train Master" the Schenectady builder switched directions back to a more traditional model with the RSD12.

The road-switcher began production in the spring of 1956 and was one of the few RS models to utilize Alco's newer (and more reliable) 251B prime mover capable of producing 1,800 horsepower (the same as the RS11). 

The locomotive remained in the company's catalog until 1963 and while it ultimately sold more than 150 units less than half of those sales went to domestic (American) railroads including the Pennsylvania, Southern Pacific, Chesapeake & Ohio, and Nickel Plate Road (the small Lake Superior & Ishpeming also purchased a few).

Tractive Effort

The RSD12 featured the highest continuous tractive effort of any Alco diesel locomotive it had offered up until that time; 78,500 pounds (continuous) that included dynamic braking. 

In comparison, EMD's SD9 provided just 75,000 pounds. The high tractive effort was actually an Alco trademark; its locomotives had gained quite a reputation for pulling more tonnage far above what the builder would rate its products and many railroads used this to their advantage employing their units in heavy-haul service (coal, ore, etc.). 

Despite the fact that General Electric was just a few years away from releasing its own line of diesels, a result of Alco's continual loss of market share to EMD, the RSD12 still used traction motors and generators built by them.

06029357236527363893879900938.jpgA former Pennsylvania RSD12, #6858, is seen here in Syracuse, New York during the very early Penn Central era on April 7, 1968. Fred Byerly photo. American-Rails.com collection.


The road-switcher was one of Alco's first to offer a low, short nose as an option although only the Southern Pacific ultimately purchased the model with such.

In general, the builder's six axle models were meant to provide a sizable increase in traction and used on light branch/secondary lines allowing the locomotive's weight to be distributed more evenly over additional axles.

While most railroads that went on to purchase the RSD12 retired their fleets within a decade or so these unique beasts were beloved by the Lake Superior & Ishpeming for their amazing ability to pull just about anything.

Data Sheet and Specifications

Alco Class606-DL-360
Entered Production4/1956 (Chesapeake & Ohio #6700)
Years Produced4/1956-4/1963
Model SpecificationDL702
Engine251B, V-12
Carbody StylingAlco
Length (Between Coupler Pulling Faces)58' 1.75"
Weight360,000 Lbs.
Dynamic BrakesOptional
Truck TypeTrimount
Truck Wheelbase12' 6"
Wheel Size40"
Traction MotorsGE 752 (6)
Traction GeneratorGT586
Steam Generator-
Gear Ratio65:18
Tractive Effort Rating78,750 Lbs. at 5.5 MPH
Top Speed75 MPH

Production Roster


Total Built = 161

Owner Road Number(s) Serial Number(s) Completion Date
Chesapeake & Ohio 6700-6709 81943-81952 4/1956 - 5/1956
Companhia Siderúrgica Nacional (Brazil) 3501 82477 12/1961
Companhia Siderúrgica Nacional (Brazil) 3502-3510 83947-83955 10/1962
Ferrocarril del Pacifico (Mexico) 1502, 1503 82921, 82922 8/1958
Ferrocarril del Pacifico (Mexico) 1504-1508 83339-83343 2/1959 - 10/1959
Ferrocarril del Pacifico (Mexico) 509, 510 83545, 83546 10/1959
Ferrocarril del Pacifico (Mexico) 511-514 83636-83639 10/1960
Ferrocarril del Pacifico (Mexico) 515-517 83968-83970 2/1961
Ferrocarril del Pacifico (Mexico) 518-520 84080-84082 3/1962
Ferrocarriles Nacionales de México (Mexico) 7400, 7401 82919, 82920 8/1958
Ferrocarriles Nacionales de México (Mexico) 7402-7408 83548-83554 12/1959 - 3/1960
Ferrocarriles Nacionales de México (Mexico) 7409-7418 83626-83635 3/1960 - 6/1960
Ferrocarriles Nacionales de México (Mexico) 7419-7423 83851-83855 3/1961
Ferrocarriles Nacionales de México (Mexico) 7424-7428 83806-83810 4/1961
Ferrocarriles Nacionales de México (Mexico) 7429-7440 83956-83967 4/1961
Ferrocarriles Nacionales de México (Mexico) 7441-7457 84083-84099 8/1962
Ferrocarriles Nacionales de México (Mexico) 7458-7472 84278-84292 8/1962 - 9/1962
Lake Superior & Ishpeming 1801, 1802 81953, 81954 4/1956
Lake Superior & Ishpeming 1803, 1804 83547, 84293 11/1959, 4/1963
New York, Chicago & St. Louis (Nickel Plate Road) 325-332 81955-81962 2/1957 - 6/1957
New York, Chicago & St. Louis (Nickel Plate Road) 333 82378 6/1957
Pennsylvania Railroad 8655-8663 82379-82387 12/1957 - 1/1958
Pennsylvania Railroad 8664-8678 82066-82080 2/1958 - 3/1958
Pennsylvania Railroad 8679 82918 4/1958
Southern Pacific 7000-7005 83640-83645 2/1961
Southern Pacific 7006-7020 83836-83850 2/1961 - 4/1961

Montreal Locomotive Works

Total Built = 10

Owner Road Number(s) Serial Number(s) Completion Date
Companhia Siderúrgica Nacional (Brazil) 3501 82477 12/1961
Companhia Siderúrgica Nacional (Brazil) 3502-3510 83947-83955 10/1962


  • Foster, Gerald. A Field Guide To Trains. New York: Houghton Mifflin, 1996.
  • Kirkland, John F. Diesel Builders, The:  Volume Two, American Locomotive Company And Montreal Locomotive Works. Glendale: Interurban Press, 1989.
  • Pinkepank, Jerry A. Diesel Spotter's Guide.  Milwaukee: Kalmbach Publishing Company, 1967.
  • Solomon, Brian. Alco Locomotives. Minneapolis: Voyageur Press, 2009.

After receiving their batch of four from Alco between 1956 and 1963 the LS&I later acquired six, second-hand RSD15 Alligators from the Santa Fe. 

During the 1960s Alco released its Century line to replace the floundering RS series.  Unfortunately, it saw no rebound in sales or recovering interest in its products and left the market entirely before 1970. 


Wes Barris's SteamLocomotive.com is simply the best web resource on the study of steam locomotives. 

It is difficult to truly articulate just how much material can be found at this website. 

It is quite staggering and a must visit!