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ALCO S-2 - Richmond, Federicksburg & Potomac No. C


Above, RF&P No. C in the yard in May 2010. Photo by Eric Kreszl.

Interesting facts

In railroad parlance, this locomotive is a "slug". It has traction motors but no engine or generator. The electric current for the motors is provided by a "mother" unit, a standard locomotive connected by cable connections to feed current to the slug. Without this power source, a slug cannot move by itself and is not therefore a true locomotive.

Diesel-electric slugs began appearing in the 1940s as manufacturers started to incorporate multiple-unit train control as standard. The first multiple unit traction control system was developed by Frank Sprague and applied on the South Side Elevated Railroad (now part of the Chicago 'L') in 1897. It allows simultaneous control of all the traction equipment in a train from a single locomotive.

Slugs are usually built by the railroads that operate them by de-powering existing cab units, although manufacturers like General Motors and General Electric have built them from scratch. The cab is generally removed to provide increased visibility for crews in the "mother" unit.

RFP 7 & C

No. C with RF&P EMD SW1500 No. 7 at Potomac Yard on January 14th, 1972. Photo
by Marty Bernard.


The Richmond, Federicksburg & Potomac bought 22 S-2 end-cab switchers from the American Locomotive Company between 1942 and 1948. No. 71 was one of the last batch of 10 delivered in September 1948 (Nos. 62-71). The RF&P then rebuilt three of its S-2s into slugs in late 1967: No. 62 as "A," No. 70 as "B," and No. 71 as "C."

Slugs allow better use of a locomotive's power output at low speeds, and most have therefore been built to use in switching yards, although some road-switchers have been adapted as slugs for mainline service. At low speeds, a diesel-electric locomotive engine can generate more electric power than its traction motors can use effectively. By increasing the number of traction motors available to the locomotive, a slug thereby increases both pulling and braking power. In addition the load on each traction motor is reduced, which helps prevent overheating. Slugs typically also carry ballast to increase their weight and improve traction.

RFP 7 & C

No. C with RF&P EMD SW1500 No. 7 at Potomac Yard on April 7th, 1977.
Photo by Tim Darnell.

The RF&P slugs spent their working life switching in Potomac Yard in Alexandria, VA just across the Potomac River from Washington, DC.

Potomac Yard opened on October 15, 1906. It was operated by the Richmond-Washington Company, which controlled the RF&P, and was in turn jointly owned by the Pennsylvania Railroad, Baltimore and Ohio Railroad, Southern Railway, Chesapeake and Ohio Railway, Atlantic Coast Line Railroad, and Seaboard Air Line Railway. Potomac Yard was intended to bring some order to the capital's mishmash of active and abandoned rail lines, and during its heyday in the 1930s, it was one of the busiest rail yards on the Eastern Seaboard.

The S-2 (the "S" stands for "switcher") was ALCO's most successful diesel-electric locomotive. Between 1940 and 1950, 1,505 were built: 1,465 at ALCO's works in Schenectady of which 1,442 were for U.S. railroads and 23 for the National de Mexico, and 40 at ALCO's subsidiary the Montreal Locomotive Works in Canada all of which were for Canadian railroads.

Most of the RF&P's S-2s were sold to other railroads or dealers in the mid-late 1960s, although a few held on into the 1970s and early 1980s. We acquired No. C
in 1983, and have since been using it to provide parts for our other ALCO locomotives.
We also have an unmodified S-2, NASA No. 1, in our collection.


Status: On Display, Not Operational.
Acquisition Date: 1983.
Built: ALCO 1948 / Converted to a slug by the RF&P 1967.
Serial: 76171.
Length: 44 feet 5 inches.
Height to Top Engine Hood: 12 feet 1 inch.
Traction Motors: Four GE 731.
Trucks: Two 4-Wheel.
Truck Wheelbase: 8 feet.
Wheel Diameter: 40 inches.
Configuration: B-B.
Air Brake: Westinghouse 14EL.

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