I read something that said WP purchased only 4 axle diesel locomotives. The reason given, was that WP needed to save money over buying more expensive 6 axle units. (?) Didn’t WP have a fair amount of grades to overcome? I would have thought that 6 axle units would have pulled better, and therefore been a better investment-no?
Hi MS,
Grades on the WP were no more then 1% (on the main anyway) so that might be a factor. Perhaps curves in the canyon may have influenced there decision but I doubt it. It’s kinda odd considering the size and axle count of the motive power used in the steam era. Just a guess but I would say the WP felt there lines were more suited for 4axle speedsters then 6 axle luggers.
Speaking of WP I just came across some news footage of a recent (last week I think) derailment on the WP main at Herlong. I think I’ll post it on a new thread though.
There was no “need to save money.” And no need to spend it unwisely either.
In the WP’s day, there was absolutely no reason for any railroad to acquire a 6-axle locomotive unless it wanted to only power trains sufficiently to obtain uphill speeds in the 11-22 mph range, and had the track, traffic, and tonnage to justify the expense of six-axle locomotives. Above 22 mph the 3,000 hp D.C. 4-axle locomotive can put all of its horsepower onto the rail, meaning it is exerting more work than the 3,000 hp six-axle locomotive because it has less of its own weight it has to move. SD40s have a minimum continuous speed of 11 mph – below that level they are into the short time on their traction motors. SD7s and SD9s had a minimum continuous speed of 6 mph, which is fine for a DM&IR or SP branch line but not for a main line with time sensitive perishables, canned goods, and autoparts.
In addition, six axle locomotives cost 25-30% more to purchase, cost 20-30% more to maintain, increase rail wear on curves, increase wide gauge on curves, require a higher standard of curve maintenance, have a much tighter FRA limit on permissable mismatch on wheel wear resulting in more frequent wheel truings and more frequent wheelset/motor “combo” swaps, and haul around 60 or so more tons of extra steel, copper, and iron that contributes nothing to train hauling at speeds greater than 22 mph but causes fuel to be burned to move it along.
WP’s maximum grade on the Oakland-Salt Lake City main line was 1.0% compensated. The High Line has 2.2% grades where six-axle power hypothetically might have been of value but given the traffic levels and fleet utilization reduction that would have resulted in trying to create two locomotive pools on such a small railroad, it never made financial sense. On a 1.0% ascending grade, if a hp/ton ratio is provided sufficient to give train speeds in the 25 mph range, then there is no need to waste money on six-axle u
Thanks Railway Man. I thought it was weird, the way the author had made it seem like a bargain hunter decision, and not an operating decision. I am now more confused, though, about the optimum uses for 4 and 6 axle units.
I thought 6 axle units were prefered for heavy pulling, regardless of grade, and 4 axle units were prefered for speed? Now, I wonder why SP had all those SD7’s, 9’s, 40’s, and 45’s. For pulling heavy loads uphill both ways? (Kinda like my folks-walking to and from school through the snow, uphill both ways.)
If my memory serve’s me, I remember chatting with my shop forman in the late 60s, at the Oroville shops about six axle trucks, being that some six axle demos were operating in the Feather River Canyon with wheel and rail tests. He stated he did not think the company (WP) would purchase due to the tight curvature in the canyon due to wear and damage to the rail and wheels due to the longer ridged truck. I dont remember if the demos were EMD or GE, as both companys wanted order for WP ageing fleet.
When UP took over and started running 6 axle trucks, I remember chatting with track maint people and all felt they were really hard on the track. UP then started a super-elevation and realignment of the canyon trackage. Also started to retrofit the tunnels for double stacked containers…John.
Now I am confused. What changed in railroading today to make the 6-axle locomotive dominant? Is it just all of the PRB coal necessitates 6-axle locomotives, so they might as well standardize?
I would think train speeds on most lines today, and certainly on the mains, would be above 22 mph, which would make the 4 axle locomotive more desireable?
Gabe
Methinks the 6 axle has become the new “GP”. Steering technologies and revised track construction standards have likely reduced the early problems that have been mentioned above.
Speed, of course, requires horsepower - which is why a switcher can move a substantial train - just not very fast. I suspect a properly geared and ballasted switcher powered by a lawnmower engine could move 100 cars. Hope you aren’t in a hurry, though.
The early SDs were nothing more than six axles under a four-axle power plant. Horsepower has now risen to the point that each of the six axles now has nearly the same HP as each of the four axles did on later high-HP four axle locomotives.
I suspect that a four-axle, 4400 HP locomotive might tend to be a bit slippery, which was a problem even the GP-40 suffered at 3,000 HP.
On a slightly different tack - this discussion makes me wonder how things might have played out on the Nickle Plate (“high speed freight”), had they continued to exist as their own entity. One might think that they would have been a heavy user of high horsepower four axle power, since speed was their forte.
Four-axle power can no longer handle the weight of modern propulsion machinery, and can no longer successfully transmit that power to the rails. The last 4-axles built (GP60 amd Dash8-40B) were very slippery inspite of being very heavy on 4-axles.
Not on the upgrades. A good example is the New York Central main line west across NY-- it apparently has a short 1% upgrade east of Batavia. To climb that hill at 22 mph you’d need 2 hp/ton or a bit more (not counting momentum) and lots of trains have less than that. They can make, say, 50 mph on the level, so they can do fine over 95% of the railroad-- but most railroads have one little hill that demands much more tractive effort. Which they obtain by allowing speed to drop below 20 mph.
To clarify about the GP40-2 – as RWM pointed out, it’s supposed to put out full power at 23 mph and up. Dunno about the GP60, but it was probably supposed to do about the same-- i.e. it’s supposed to match an SD60 above 23 (?) mph. Question is, how often does it live up to that. On paper there are lots of trains that “should” be powered by high-horsepower B-Bs, but they’re not-- and one has to wonder if that’s due to bitter experience.