A1A vs. C-C

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David914

Active Member
#1
Can anyone tell me what advantages or disadvantages there were to employing A1A trucks over C-C trucks? For that matter, why didn't Commonwealth trucks provide a good ride as compared to drop-equalizer trucks?
 
#2
A1A gives better support, for either heavier load or same load-lighter rail, than a B truck, when there is not more power available than can be used by two traction motors on a truck.

Bruce
 

Bob

Forum Host
Staff member
#3
Exactly, better load distribution. To use a C-C configuration, you have to have enough power available, and you also need to have a reason for all of that power. Sometimes you don't have one or the other, but you do have a situation where the 6 axles trucks help distribute the load. Of course the downside is that the 2 idler axles not only don't contribute any pulling force, they also take some of the weight off of the driving wheels. If the weight distribution is equal on all 3 axles, then you only have 2/3 of the weight on the drivers that you would have with a 4 axle unit of identical weight. Of course it's usually not quite that simple, but the basic concept works.
 
#4
What's less obvious is the move from DC traction to AC. North American railroads are mostly ordering AC traction now, a historic shift. Six axles to spread the weight, but only four AC motors which holds down the purchased cost and greatly reduced maintenance from not having commutators and brushes, and almost unlimited full power traction. The other trick is the lifting center axle to more heavily load the powered axles when wheelslip is detected.

Wonder if this practice is cracking or even breaking rails?

I'm sure there are still plenty of six powered axles units being ordered, where maximum traction needed in mountain service.
 

Bill Anderson

Well-Known Member
#6
I believe the term we are looking for is axle loading, or the weight of the loco (or car) divided by the number of axles. Track, bridges, trestles, and similar structures all have maximum axle loadings. Some B-B trucked lcoos have too high an axle loading for certain structures. A1A-A1A trucks distribute the weight of the locomotive over six axles instead of four, resulting in lower axle loadings.

For that reason, A1A-A1A trucked Baldwin and ALCO road switchers were popular with short lines and class I railroads with branch lines that had light rail, bridges, and trestles with low axle loadings and no need the tractive effort of a (presumably) more expensive C-C unit. There were no similar offerings from EMD in the 1940's and 50's when most North American railroads were converting from steam to diesel locomotives.

GMD of Canada built the Canada-only A1A-A1A GMD1 from 1958-60 for lightweight prairie branch lines.
https://en.wikipedia.org/wiki/GMD_GMD1

The Milwaukee Road ordered the lightweight, custom built C-C SDL39 from EMD in 1969-1972 to deal with light axle loadings on their branch lines while getting the tractive effort of six traction motors.
https://en.wikipedia.org/wiki/EMD_SDL39
 
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Bill Anderson

Well-Known Member
#7
A late follow-up. Many of the early passenger diesels had A1A-A1A trucks to distribute the added weight (and lessen the axle loadings) of a steam generator and water. Fairbanks-Morse had a series of passenger diesels that had a rear A1A truck and a front B truck for a B-A1A wheel arrangement.
 

BNSFEng

Locomotive Engineer!!!
#8
What's less obvious is the move from DC traction to AC. North American railroads are mostly ordering AC traction now, a historic shift. Six axles to spread the weight, but only four AC motors which holds down the purchased cost and greatly reduced maintenance from not having commutators and brushes, and almost unlimited full power traction. The other trick is the lifting center axle to more heavily load the powered axles when wheelslip is detected.

Wonder if this practice is cracking or even breaking rails?i

I'm sure there are still plenty of six powered axles units being ordered, where maximum traction needed in mountain service.
Actually, with BNSF converting the 600 series C44-9Ws to A1As and AC traction, there are fewer and fewer 6 axle powered locomotives. BNSF stopped buying 6 powered axle engines this year when they stopped making the SD70ACEs. Mainly, GE sells mostly the new Tier 4 engines that are A1A A1A ACs to class 1s, but they do offer it in C-C configuration and in DC.
 
#9
I have a similar question. I know that the early Amtrak designs (GE E60s, P30CH, and EMD SDP40F classes) were notorious for unreliability and derailments with their C-C trucks. Keep in mind that passenger train length was greatly decreasing. However, with first generation diesels like the PAs, E-units, and Eries, they didn't seem to have much trouble exceeding 100 miles per hour with A1A-A1A trucks. Why was there such a difference with different six-axle types. My theory is that because there's no motor on the middle axle, A1A trucks would be shorter in length, and therefore less vulnerable to derailment than C trucks. Can anyone else clarify this?
 



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