Mookie’s GP/SD question reminds me of a question that I have been pondering for some time. Are 4-axle locomotives becoming extinct? I know the GP-60 sold some, but did it sell a lot and is there a successor planned? Does GE even make a 4-axle unit?
Is there any advantage of having a 4-axle unit on a well maintained main line with no sharp-radius curves?
It seems to me as though the 4-axle unit is becoming extinct outside of yard work, can someone tell me how accurate my assessment is?
See the thread I just posted over there in Mookie’s “Why?” thread…
The advantage of the four-axle unit is that it has fewer axles, and less associated cost (e.g., 4 traction motors instead of 6 to stock and maintain, less mass in the truck frames, etc.)
There is still an advantage in very-high-speed service to having two-axle power trucks, in part because the mass of the required sideframes can be less for a given set of guiding characteristics (and the additional mass required for steerable outer axles adds up too). Theoretically a two-axle truck is a bit easier to cross-equalize.
Aside from that, there’s a clear advantage geometrically in going to a three-axle steerable truck (such as HTCR-II or FlexiFloat) – on a main line with transition spirals and “no sharp-radius curves” the effective rigid wheelbase of such a truck is zero. Much more difficult to make this work with a two-axle truck that has a pivot to the carbody. Bolster design, secondary suspension, etc. generally better. Also better weight distribution on the axles during acceleration or braking…
Not much call for power per axle above what existing, stocked designs of TM are putting out. That severely limits the ‘viability’ of high-horsepower 4-motor units… and about the only place it would pay to build such units ‘new’ would be in very fast intermodal, or passenger, service.
I think the real point is that the four-motor unit market is quite nicely supplied by the used-equipment market… both with respect to anticipated needs and to a source of low-cost parts for ‘new’ locomotives (cf. the Green Goat). Seems that the ‘minimum’ power for RoadRailers is a six-motor unit, and for other mainline trains either a pair or triple of six-motor units. “Road slugs” might benefit from two-axle trucks, to save weight or length, but… see above; nobody buys slugs new and keeps their job for long!
The only decent 4 axle GE units are the P-42 which is designed for passenger service. GE sells nothing else from what I have seen on GE’s website. I would imagine that the P-42 would be excellent roadrailer power.
Please correct me if I’m wrong(which I probably am), weren’t 6 axles developed
to cut down on wear on lighter trackage?To distribute the weight more evenly?
NS uses GP60’s on four intermodels around here, while they were designed for RoadRailer service I have yet to see one on a RoadRailer and I’m sure I never will.
EMD doesn’t even list a high horsepower 4 axle road switcher to date and according to the Field Guide To Modern Diesel Locomotives, the last GE 4 axle Dash 8’s were build the spring of 1992 for the Santa Fe
No, not high horsepower, if 1500 or 2000 hp is considered a low hp, but UP has a fleet of both, leased from CEFX running all over the place down here, you see them routinly hauling 100 car freights all the time, at track speed.
There’s also a lot of shortlines that have weight restrictions on certain wood trestles and other spots on the line. So I’m sure 4 axle stuff will be around for sometime, even with the proliferation of used SDs out there. Which is good, because I’ve always liked the geeps.
I too am sure that 4 axle locomotives will be around for a while. However, don’t think there will be any high horsepower 4 axle locomotives built in the future, unless they can get more of that horsepower to tractive effort.
On a philosophical note - Who uses the 4 axle units? The folks who keep traffic supplied to all the 6 axle units. The extinction of 4 axle units may well bode ill for the world of railroading.
There are enough 4 axle locomotive in existance to serve the industry for the forseeable future. What fuels new locomotive purchases for line haul service is improved fuel efficiency and a reduction in fleet size. For example, an SD70 is about 20% more fuel eff. than an SD40-2 and you can replace 4 SD40s with 3 SD70s (more or less). That generates a decent return on investment such that it’s cheaper to buy new than rebuild the old.
4 axle units in light duty service, such as local or branchline work, don’t burn enough fuel that saving 20% of it would amount to much. Also, it is not likely that there would be much of a replacement ratio, either.
This being the case, it is better to just maintain or rebuild in kind than purchase new. You can keep a GP28-2 going almost indefinitely. There may still be some running after we’re long dead!
I swear by my copies of Diesel Locomotives: The First 50 years, The Contemporary Diesel Spotters Guide ( 2nd edition ), the Field Guide To Modern Diesel Locomotives and from the other guys The Contemporary Diesel Spotters Guide ( year 2000 edition ). These are some of the best quick reference books out that I’m aware of.
