I’m sure this has been addressed before, but I couldn’t find anything on it. I have been perusing Gerald Foster’s ‘A Field Guide to Trains’, and noticed that there are some locomotives, such as the SD60 and GP60 that have the same horsepower, although the SD is 12 feet longer, and the GP38 and SD38 likewise.
I presume that the extra length on the SDs is due to the 6-wheel trucks, but is there another reason why roads purchased SDs over GPs or vice versa? Is there an advantage in tractive effort with the 6-wheel trucks? Are the GPs cheaper (not in construction, but in price) than the SDs? Are there extras in the SD package that don’t fit on the GPs?
The latest trend is away from four-wheel trucks, probably because the latest and greatest engines are so heavy.
In general, SDs were used on poorer track (the wieght of the locomotive is spread over six axles instead of only four), and for situations where tractive effort was more important than speed. Hence many SD units worked on coal trains, hump yard humps, and drag freight trains. Some had extra ballast applied to add weight (and tractive effort) to the engines.
Most SD units that I have seen over the years look like they have extended platforms fore and aft of the body compared to geeps. Now that I think about it, over the years, I have seen more SD units than geeps running on the former C&NW New Line, and the former Wisconsin Central line through my hometown.
That’s part of it (weight and axle). Also, you can do more with less (it also MOST IMPORTANTLY saves money) Mine shifers around here (former C&O) are pretty much all 6-axle GE stuff. One of these can do the work of 3 smaller 4-axle units.
The following is my understanding from the topic from reading John Armstrong’s book “The Railroad ,What It Is, What It Does”
Tractive force is limited by engine horespower over most of the diesel locos speed range so that a 4 axel and a 6 axel are nearly the same with the 4 axel able to haul a little more tonage due to it being lighter (not needing as much power just to move its self.)
At low speed however adhesion becomes a factor. To keep the loco from slipping power has to be reduced. The higher weight and the extra traction motors (6 axel has 50% more traction motor thermal capacity) allow the 6 axel to use higher horsrpower at lower speed and therefore haul a heavier load (about 50% more) on long or steep grades than a 4 axel can.
So 6 axels used in mountains and 4 axels in flat lands.
Some railroads chose one type however no mater what the terrain. The Western Pacific for instance had no 6 axel locos. They apparently believed that 4 axels were less distructive to the track structure and were less expensive to maintain. The grade in the Feather River Canyon is only 1%, but it is a looooooong one.
Track curvature can also be a factor. When Rio Grande standard-gauged the Monarch Branch in the 1950’s, they originally planned to assign an SD7 to the branch. The long rigid wheelbase of the SD7 was too much for some of the tighter curves (derailments were not uncommon) so GP9’s were assigned.
In discussion with my local N-scale group, it was brought up that the 6-axle SD would have greater tractive power due to the extra axles, but also when traversing rail and bridges that weren’t designed for large locomotives, the weight of the locomotive would be spread out over a longer span.
It was also brought up that the GPs had a greater acceleration than the SDs, but not necessarily a higher top end speed. In other words, the GP could get to say, 50mph faster than the SD, but the SD could pull more cars at 50mph than the GP could.
In response to CSSHEGEWSCH’s note concerning track curvature and locomotive wheelbase, that was also pointed out to me as a reason for keeping GPs on the road.
Is there a trend toward wholly adopting the 6-axle locomotive for mainline use and relegating the GPs to yard or way duty?
Thanks for all of the replies. Makes it so much clearer to me now. So much to learn…but that’s why I ask.
Something additional to consider is that in some cases the third axle was not powered. It was used only to spread the weight over more wheels. I believe, but a I am not positive that the early SD’s and E units only had power to 4 of there six axles.
E’s had an idler axle, all axles on an SD are powered. There were several road-switchers on A1A trucks for light-rail branches, such as the RSC2 & RSC3, AS416’s, some GMD1’s, etc.
The SDL39 was also designed for light-rail branches, but had all powered axles. It is basically an export design modified for North American use.
Dear CSSHEGEWISCH,
The Alco RSC1 also had an A1A truck (of course, you probably guessed that they would anyway [:)]). I volunteer at a museum (Orange Empire Railway Museum) with an E8 and two RSC1’s, and so I see them relatively frequently.
And now, the railfan side of me is speaking: those six wheel trucks look better, although I know most railroads don’t really care.
In DC traction motored locomotives an SD could not reach a higher speed than a GP (assuming equal horsepower and gearing of course). Any DC motor starts generating a back voltage whenever it rotates. This back voltage is proportional to the speed. So at speed the GP must overcome the power needed to move the train plus the “anti” power generated by its four motors. An SD must overcome the same train resistance plus the back voltage of SIX motors. So with an equivalent train the GP can reach a higher speed than can the SD. But at lower speeds the SD is able to apply more of its horsepower to the rails without losing traction.
The counterelectromotive force (back voltage) generated by DC traction motors is the reason that transition steps exist (Series, series-shunt, parallel, parallel-shunt). Somebody with a better knowledge of locomotive electricals please help.
