RAILforum: Two Axle or Three Axle Trucks

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Posted by Konstantin (Member # 18) on :

In the early diesel years, typical passenger locomotives had three axles, and freight locomotives hade two axles per truck.

On my last trip aboard Amtrak, I noticed that almost all of the BNSF freight locomotives we saw had three axles, and all of the Amtrak locomotives had two axles.

It seems that there has been a complete reversal between two and three axles. Does anybody have any ideas about why?

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Elias Valley Railroad (N-scale)
www.geocities.com/evrr

Posted by Eric (Member # 674) on :

I think part of it may be that freight trains are getting longer and heavier, and the locomotives need more axles/wheels to increase tractive effort.
Amtrak had several derailments in the 70's with their SDP40F units (three-axle) when they were used at high speeds, and a few freight carriers banned the units (others imposed speed restrictions). Amtrak ordered the F40s as replacements, and they had two-axle trucks.
So, at higher speeds, two-axle trucks must take the curves and grades better, and for slower speed and heavier freights, there is less of a risk for derailment, so they can use the three-axle trucks. They can also use fewer units to power each train because of the increase in tractive effort. Hope this helps!

[This message has been edited by Eric (edited 08-17-2002).]

Posted by PullmanCo (Member # 1138) on :

Let's talk about trucks for a moment.

Any of Dr Cynthia Priest's books (SF Diesel V1 or V2, UP Diesel V1 among others), Joe McMillan's 1970s work on Santa Fe's Diesel Fleet, or the Kalmbach Locomotive Cyclopedia will discuss truck design in some detail.

Early E units (EA-E9) as well as the ALCO PA's of the 30s-60s had A-1-A trucks (loco config was A-1-A/A-1-A). That meant a powered axle, an unpowered idler, and a powered axle per truck.

The entire F family of EMD products, the GP family, and the GE U##B units have B trucks (config was B-B). Two axles, both powered.

Union Pacific's one and only DD40AX Centennial fleet had D-D trucks... each truck received power solely from one of the two prime movers on the unit!

The SD family, including the infamous SDP40F has C trucks. Three axles, both powered. The original reason GM designed the SD family was to have a lower loading weight per axle for branchline service.

These days, though, with 6000HP single units, I'm not at all surprised that we rarely see B-B designs anymore.

John

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The City of Saint Louis (UP, 1967) is still my standard for passenger operations

Posted by atsf3751 (Member # 1538) on :

While the majority of early passenger diesels were six-axle units (E's and PA's), they had only two powered axles per truck in an A-1-A configuration as mentioned above. Although I'm not 100% sure, I have heard that this configutation was more stable and made for a smoother ride.

C trucks on freight units did not become popular until the 1950's, and even then were considered largely for "special duty" operations, hence EMD's entry into the six-axle freight market with the SD7. However, six axle units were long considered to be only for lugging heavy drags and four axle units were for light, speedy trains like intermodal. That has all changed today.

In the late 80's and early 90's, Santa Fe placed a number of orders for high-horsepower four axle power (notably, GP60M's and DASH 8-40BW's) but then quickly went over to six-axle. Why? Simply put, the extra two axles and longer, heavier frame provide more tractive effort. Most crews would agree they ride better too.

Passenger trains today are virtually all-four axle because they need lighter, speedier locomotives. A six-axle DASH 9 may be able to accelerate to 70 mph, but a four axle F59 with a light train can do so much faster.

Posted by Konstantin (Member # 18) on :

Thank you for the replies. Thank you ATSF3751 especially. You have answered my question directly.

I appreciate the information from all of you.

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Elias Valley Railroad (N-scale)
www.geocities.com/evrr