CN: 4-8-4 vs 4-8-2

In the string on Boston & Maine's R1 4-8-2 (which considers the question whether B&M would have been better off with a 4-8-4), there is a bit of a digression about Canadian National's 4-8-4 and 4-8-2 classes. I expressed a bit of surprise that CN went for a 4-8-2 (the 6060-6079 "Bullet Nosed Betty") in 1944 AFTER a long period of buying 4-8-4 (their final 4-8-4, the 6200-6264 series, were built in 1942-1944). Other people -- notably Pneudyne -- set me right by pointing out reasons they might have done this: go read that discussion before finishing this one.

What I only recently noticed (in Drury's "Guide to North American Steam Locomotives") is that CN had, by that time, had a lot of experience comparing the two types. Their earlier 4-8-4 (6100-6189) had been built in 1927-1930, and their earlier 4-8-2 (6000-6058 (with two built for subsidiary GTW)) were built in 1923-1930. So evidently in 1929 (?) they ordered BOTH types, having six years of experience with 4-8-2 and two with 4-8-4. So: they -- with a fair bit of supporting evidence! -- concluded that both types had a role, mainly in different services.

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CN seems to have thought of its 4-8-2 as passenger power, with the 4-8-4 primarily used on freight (though both types could, in a pinch, fill in on either sort of train). My current guess (my current effort to fill in details of what others have tried to explain to me is this):
---With similar cylinders and drivers and boiler pressure, either type can accelerate a train from a stop.
---At high speeds, a passenger train doesn't need as much power as a 4-8-4 can produce: so a 4-8-2 will do the job.
---At medium speeds (high speeds for freight!), freight trains need more power: so a 4-8-4 is better.
But of course, even if this is right, it's only a first approximation. After all, many railroads (looking at you, B&M; looking at you New York Central) were happy to use big 4-8-2 on freight!

I suspect that it was mostly a matter of matching the locomotive power curve to the haulage and speed requirements.

If you have access to the book “A practical Evaluation of Railroad Motive Power” by P.W. Kiefer, then the various charts in it provide a good illustration. For example, the NYC L-4 4-8-2 may be compared with the S-1 4-8-4. Where the haulage and speed requirements were within L-4 capability, then that was probably a more economical solution than an S-1. I suspect that CN may have done the analysis for some of its passenger haulage requirements with the finding that an updated 4-8-2 would indeed be a good match for the job. In a relatively small fleet, standardizing on a dual-service 4-8-4 might have been prudent, but in a large fleet, such as CN had, there was probably minimal utility penalty incurred by also fielding a lighter 4-8-2 design, meaning that its lower operating costs could be taken advantage of.

If one looks at eight-coupled road locomotives as a group, then the reasonably feasible wheel arrangements were 2-8-2, 2-8-4, 4-8-2 and 4-8-4. The number of carrying axles used was partly determined by the power, and thus the boiler size required. Speed was another determinant. For locomotives that habitually operated above say 50 mile/h, the four wheel pilot truck was preferable. For a given weight on drivers, possible boiler sizes were smallest for the 2-8-2, somewhat bigger for the 4-8-2, a little bigger again for the 2-8-4, and with another increment to the 4-8-4. Driver sizes varied according to desired top speed and where one wanted to place the peak of the power curve relative to the modal operating speed. But empirically it would seem that with the 2-8-2, there was little to be gained by going beyond around 63 inches. On the other hand, the 2-8-4 justified going to 69 inches or thereabouts, but not beyond because the limitations of the two-wheel pilot truck. The 4-8-2, not being speed constrained in this way, could go to around 73-74 inches where desired. The 4-8-4 could go a step further to around 80 inches for the fastest requirements, although it was fine at around 69 and 73-74 inches where those met the speed requirements.

It would be interesting to see the power curves for the CN late 4-8-2 and 4-8-4 designs. I suspect that they would be closer together than was the case for the NYC L-4 and S-1. But there might have been some convergence at the upper end of the speed range, if say the final 4-8-2 had been specially configured to have a shallower droop down from the peak.


Cheers,

I have extracted some of the charts from the Kiefer book as general illustrations, below.

The L-4 4-8-2 was probably reasonably representative of the late 4-8-2 as a species, perhaps even a little better than average when its weight and grate size are taken into account. It was more powerful than the J-3 4-6-3, but its key advantage was probably less its greater power than its higher adhesive weight that allowed reliable starting and acceleration under pretty much all ambient conditions, and without the need for a booster.

To my eye anyway, the charts seem to show that if you wanted, and could justify on fleet utilization grounds, a fast passenger locomotive for low-grade routes, but you also wanted a boosterless design with more reliable adhesion than a 4-6-2 or 4-6-4 then a 4-8-2 was a logical and probably ideal choice. There would have been no need to go to a 4-8-4 unless the locomotive was also required for freight service, in which case the extra mid-range power would probably be useful.

Of course, in steeply graded territory, the extra power of the 4-8-4 would be worthwhile, particularly in the mid-range.

Also apparent from the charts is that for the NYC, the S-1 4-8-4 was really overkill – perhaps developed more as a prestige project than to meet a real need. (The design improvements that allowed higher monthly mileages, etc, could equally have been applied to a 4-8-2.) It did offer better acceleration, but that may have been a byproduct rather than a specific design objective design. In the event, NYC found that 4000 hp, two-unit diesels could do the heavy passenger job reliably in terms of point-to-point speeds.

If nothing else, the charts show that in terms of peak power, steam locomotives had to be significantly overpowered in order to have enough elsewhere along the speed curve. Nonetheless, recording very high peak power outputs seemed to be an obsession with some railroads, and even more so amongst observers. In that vein, kudos to the UP who didn’t seem to care much about such matters. Rather it designed its late steam locomotives to undertake, and to do well, their particular haulage tasks, with their power curves being a consequence, not a cause.

The 4-8-2 seems to have suffered somewhat, at least in the observer community, from not being a “superpower” locomotive, disqualified from that club by its lack of a four-wheel trailing truck. Yet it was the wheel arrangement of choice for some roads that had previously dabbled with the 2-8-4, including the B&M and the IC. And whilst the MoPac rebuilt some 4-8-2s in kind, its 2-8-4s were rebuilt as 4-8-4s.



Cheers,

And one more attachment.