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The Alco C643H

(Please note that the below models are not that of the C643H.)

The Alco C643H was one of the builder's most powerful diesel locomotives. Built for the Southern Pacific the model was similar to the Union Pacific's C855 design in that it was essentially two locomotives merged into one. At the time both SP and UP were experimenting with large, powerful single unit diesel locomotives and both companies had been quite loyal customers of the American Locomotive Company (Alco) for decades, dating well back into the steam era. The SP's version of Alco's powerful diesel was somewhat different than UP's although both employed multiple prime movers to achieve their high horsepower output (although SP's was not quite as large). Also like the C855, the C643H was plagued with problems and Southern Pacific had retired their fleet of three by the 1970s. UP's version lasted no longer and was also retired at around the same time.

The Alco C643H, classed by the company as its DL-643, was produced in the early fall of 1964 as the builder manufactured a set of three for Southern Pacific who, like rival Union Pacific, often tested big horsepower in an attempt to see if they could reduce their motive power per train. The SP took this a step further by testing diesel hydraulic designs as it also purchased twenty one Krauss-Maffei model ML-4000s to see how they performed in freight service. The C643H could produce 4,300 horsepower using two 251C model prime movers rated at 2,100 horsepower each. Unlike the C855 model which used four B-B trucks, the C64H used two standard C-C trucks and weighed much less at only 378,000 pounds (interestingly, at 189 tons the model even weighed less than its six-axle Century line which topped out at over 200 tons).

As you might expect, the C643H was not a standard model in Alco's catalog although the company would certainly build any type of custom locomotive a railroad may have been interested in. Internally, the C643H was basically two C628s merged together. The model used traction motors (model 752) from General Electric although its main generator was produced by Voith Hydraulic. Additionally, its air components came from Westinghouse (brakes and compressor). The locomotive was one of the longest the company ever produced (the C855 ranked first in this category) at nearly 75 feet, 10 inches in length and was capable of producing an incredible 95,000 pounds of tractive effort, on par with what some of the large steam locomotives were capable of.

Only three C643Hs were ever built for Southern Pacific, originally numbered #9018-#9020 (the railroad later classed them as #9150–#9152 and finally #9800-#9802. Unfortunately, the C643H was not particularly successful. First, the diesel hydraulic transmission performed poorly and due to the fact that it was foreign-made meant that parts were expensive and took time to arrive. Additionally, the Alco C643H, as with the C855, had not been properly tested meaning that when they entered service numerous problems and trouble-shooting issues arose, particularly with the two prime movers functioning as one. Along with everything else, crews came to loathe operating the units as they offered an incredibly jarring ride.

These factors resulted in the Southern Pacific operating their three C634Hs for less than ten years as all were retired from the fleet by 1973. Of note, UP had scrapped their fleet of C855s a year earlier in 1972. For an idea of what one of these beasts looked like please click here. For technical data on the Alco C643H please click here. Also, for more information about the C643H and all Century series designs please refer to the chart below.

Please Click Here To Return To The Main Diesel Locomotives Section

Century Series

Model Type	Units Built	Date Built	Horsepower
C-415	26	1966-1968	1,500
C-420	131	1963-1968	2,000
C-424	190	1963-1967	2,400
C-425	91	1963-1966	2,500
C-430	16	1966-1968	3,000
C-628	181	1963-1968	2,750
C-630	133	1965-1969	3,000
C-643H	3 (Built For SP)	1964	4,300
C-636	34	1967-1968	3,600
C-855	3 (Built For UP)	1964	5,500

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. If you’re interested in classic Alcos, or diesel locomotives in general, this book gives an excellent general history of both. If you're interested in perhaps purchasing either (or both) of these books please visit the links below which will take you to ordering information through Amazon.com, the trusted online shopping network.

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