I don’t think it’s all that unusual for two different railroads to examine
the same data and reach different conclusions regarding the economics
of one technology vs. another. Part of the answer may well reside in
what Michael Sol wrote above. I’ve also read that NS feels that dc motors
are more reliable and less subject to breakdown & expensive repairs. I
wouldn’t know.
But another similar contemporary example immediately springs to mind:
when ordering Gevos, dash 9s, etc., I believe the western class one roads
have usually elected the 4400 hp option. NS, for a variety of similar reasons,
has decided to stick mostly with the 4000 hp versions.
I don’t believe there is a “right” or “wrong” in either case.
MS mentions speeds - it’s possible that BNSF needs the low speed pulling capability enough that it’s worth their while - perhaps cheaper in the long run than maintaining/using pushers. Possible scenario - hard pull out of a valley followed by relatively flat running. The speeds would average out.
NS may opt to put more power on to raise the minimum train speed (a necessity with DC anyhow).
Too, we can’t really have a meaningful discussion without looking at things like average train length/weight and overall grades.
About 10 years ago, I asked an NS middle-to-upper level Mech Dept staff guy, “Why no ACs?” The main reason was that “NS ran mostly two units per trains, so no unit replacement benefit with AC”. As it turns out, that really isn’t a true statement. There are lots of applications on ACs would give good unit replacement ratios while still keeping HP/ton at required levels. I suspect the truth was that the Mech Dept couldn’t be bothered to do the training and the hassle of keeping the fleet segregated was more than anyone wanted to take on, and now there’s a lot of inertia and, perhaps some crow to eat if ACs were to be purchased.
I think some of this same “logic” is the reason NS has derated 4400 HP locomotives…
…and which RR was last to dieselize, and which RR was last to purchase GP9s, etc, etc.
Oh. . .maybe this is a fair analysis, maybe not. One thing’s obvious, though,
if you look at how railroads behave, rather than what they say:
Runthrough power is becoming more and more common every day. UP & BNSF
4400ACs are now a commonplace sight on NS and CSX roads; by the same
token, a lot of NS catfish (
Mookie BNSF splits their orders between AC and DC locomotives. The new BNSF locomotives numbered in the 7xxx series are DC motored, while the new 6xxx series are AC motored. Currently I think the scorecard goes like this.
BNSF splitting between AC and DC
Only buying DC
CN and NS
Only buying AC
UP, CP, and KCS
Jury still out on CSX because they got into the same problem that UP did, the severe need to replace large numbers of older increasingly unreliable power as quickly as possible. Once UP accomplished that they went back to buying exclusively AC motored power. Indications that I have heard is that the ES44DCs have satisfied the urgent need for more reliable power on CSX and future orders will be for AC motored power only.
2006 Actual production by builder and railroad, note because this is actual production it doesn’t match orders. Some orders partially built in 2005 or 2007.
As best I can gather from the company magazines, through “iffy” tangential references, BN has about 1,000 AC locomotives, and 5,300 DC locomotives. The AC’s appear to be almost entirely coal service, and recent purchases have been directed to that type because of the coal demand.
For those that don’t have that issue handy-- it just points out that 4400 hp AC GEs are rated for 2900 tons apiece up Cranberry, which is a lower hp-per-ton than their predecessors. Nothing beyond that.
For something beyond the basic numbers, you need to read the article. Its basic premise – in the context of our discussion here – is that a railroad will have a sufficient number of units in a given consist to produce enough horsepower to move its train at whatever speed is desired across its route; however if the railroad has to add even more units in order for the consist to produce enough tractive effort to keep the train from stalling on one or more short segments of that route, the railroad is probably wasting horsepower and should at least consider AC traction.
Folks, I am not trying to prove anything with the following statement,
so don’t jump all over me. But it is interesting to note that the two
most profitable class one railroads (in terms of the best operating
ratios) are the two roads who have chosen to align themselves with
DC power.
All you’re saying (so far) is that moving a given train from A to B in a given time requires both X rated horsepower and Y continuous-or-whatever rated tractive effort. If two GP9s produce enough horsepower but not enough TE, you can switch to SD9s. If two GP40s don’t produce enough TE, you can switch to SD40s. Nothing new there. But now there’s no such thing as a B44-9, so you could say the DC C44-9 is the starting point, and the only way to get more TE is to go AC (unless GE starts selling D44-9s, or C33-9s-- or C22-9s).
That’s correct, the issue is continuous tractive effort, with the emphasis on “continuous”. Regardless of whether the locomotives in a DC-traction consist are four-motored or six-motored, the consist has to have enough horsepower to keep train speed from dropping too far below the units’ minimum continuous speed for too long a period of time. As a practical matter, that’s not a consideration with a consist of AC-traction units. If a DC-traction consist encounters a grade that causes train speed to decrease too much, the units will derate themselves to avoid damaging their traction motors. An AC-traction consist won’t do that. So since an AC-traction consist doesn’t have to maintain the speed levels that a DC-traction consist would have to maintain, the AC-traction consist doesn’t have to have the level of horsepower that the DC-traction consist would have to have.
That’s an interesting observation. To the extent that higher average operating speeds means less congestion, greater capacity, higher efficiency and greater productivity, the higher investment in a machine designed to return that investment only if used at very low operating speeds does seem to present a bit of a conundrum regarding investment in AC power.
In the example offered by my industrial engineering colleague above, put in some numbers:
DC, 3000 hp, $1,500,000/unit, 5 per train: 15,000 hp. At 7%, total annual financing cost (P+I) of motive power on an 800 mile railroad running 18 trains per day with 120 mile divisions: $118,342,969.
AC, 4400 hp, $2,300,000, 3 per train: 13,200 hp. Total cost same railroad configuration: $108,875,531. It’s cheaper to go AC.
If there are two hypothetical “slow spots” that need the 15,000 hp of the DC engines to maintain sufficient speed to avoid overheating, what does the configuration look like by reducing the DC train power and adding two helper districts?
In that instance: DC, 4 per train. 12,000 hp. Total annual financing cost, including 2 helpers – two units each (and crew at $94,000 salary including benefit costs, two man crews, three crews per 24 hr cycle), total hp on grade of 18,000 hp, total annual financing cost (plus extra crew): $96,591,328.14. Cheaper to go DC.
The addition of helper districts saves, in that instance, $12,284,203.13 in annual financing charges over the cost of AC equipment, especially where the helper districts apply considerably more hp to t
That might be true in some circumstances; but let me change the scenario to reflect CSXT’s circumstances. Instead of discussing “slow” spots, we need to discuss “helper” spots. In other words, we’re dealing with route segments on which helpers are going to be required regardless of whether the locomotives in use are AC-traction or DC-traction. That’s because a train of significant length can’t be moved across the curves and grades of those segments without employing a helper to reduce in-train forces. So helper-related expenses approach being fixed costs. In that kind of high-cost situation, it seems particularly appropriate for the railroad to maximize the length of its trains as much as the capabilities of the assigned locomotive consists will allow. And because of the AC-traction characteristics that we’ve been discussing, the trains can be longer if AC-traction units are assigned to them than they can if an equivalent amount of DC-traction horsepower is assigned to them.
I guess this brings us back to the issue of whether smaller faster trains are more profitable than longer slower trains. CSXT believes, with regard to its tonnage traffic, that length is more important than speed. That’s not saying that speed is irrelevant; it’s just saying that there are financial advantages to including, in tonnage trains, the additional cars that AC-traction locomotive consists allow CSXT to add.
In other words, if they don’t have enough continuous TE they won’t make it. True nuff.
In other words, AC units can maintain full horsepower down to a lower speed. True nuff.
I guess that’s the first question to ask, when wondering whether AC makes sense over a given piece of RR: if your trains climb the ruling grade at 7 mph, will your crews cover their run in 12 hours? If not, forget AC.
One aspect of the coal-hauling business I don’t see being discussed here that the ACs are ideally suited for- the loading methodology. It’s my understanding that coal drags are loaded in the PRB by running the consist at a very slow speed (one MPH?) under the loading chute. Maybe NS doesn’t have any coal loading facilities like this?
The subscription DVD which is being sent by Trains to subscribers, in an attempt to get them started in a DVD of the month club (Discussed in this thread) is called ‘Ultimate Railroading DVD Series: Big Power’.
It has a nice discussion of the merits of AC vs DC power.
It sounds to me a little like the Ford vs Chevy argument for some engineeering departments…