Continuing the discussion on various comments:

North American diesel-electric practice started out with the B-B wheel arrangement (with a few exceptions). As the passenger diesel was being developed, some models needed the ability to carry the weight of dual prime movers and/or train heating/cooling boilers and enough water capacity to supply them. This resulted in the A1A-A1A arrangement becoming common in passenger service. In the western US, the EMD E models fell out of favor because the weak traction motors and fairly high minimum continuous speed caused many railroads to favor the Alco PA or EMD F units instead. Also EMD did not offer dynamic braking on the Es until the E8.

The other use of the A1A-A1A was where light rail and bridges required a unit with lighter axle loading. In the early days of dieselization, 25-30 short ton axle loadings were typical. There were still plenty of smaller railroads and branch lines that still had 60-75 pound rail and E40 bridges that were still economically viable. For these lines, railroads either purchased light B-B units (such as the GE 70 ton); or A1A-A1A designs that were typically the same weight of a B-B units spread across six axles. Of course, a disadvantage of an A1A-A1A over a similar B-B of the same weight is that the A1A only has 2/3 of the weight on powered drivers so its starting tractive effort and adhesion is proportionally reduced.

The first C-C (six axle six motor) units were designed in part to lower the axle loading and in part to lower the amperage load on the individual traction motors. Early Baldwin, EMD and Alco C-C units typically weighed about 165 tons or 5,000 pound axle loading less compared to a typical B-B road unit. Railroads soon discovered that by ballasting them up to the maximum axle load a C-C unit would have about 50% greater starting and continuous tractive effort compared to a comparable B-B design. The larger C-C design also allowed for more options including more fuel to be carried without encountering weight limitations. But the basic fact is that under most conditions above about 15 mph the tractive effort advantages of a C-C compared to a B-B are basically lost. Some railroads such as the DRGW (before the coal boom), WP and many eastern railroads found little advantage in having six axles because their desired train speeds were high enough not to require six motors.

The basic weight of an EMD six axle for years was listed around 180 tons and some railroads such as CSXT continued to buy engines at that weight. In recent years, the trend has been to ballast engines to the maximum limit to take advantage of the added adhesion. Where it has been historically typical for the engines to have a greater axle weight than a freight car, with the increase to 143 ton cars as the current standard then a six axle engine at 215 tons isn't really that heavy because it has the same axle loading as the loaded freight cars behind it.

The last specifically light axle loading locomotives for the US I can think of where Milwaukee's SDL39s for working their light rail branches. Many operations outside North America had far more restrictive axle loadings making the use of light designs and additional axles more common. In the case of narrow gauge, the extra axles aren't always to spread the weight but to provide enough traction motor capacity to fully use the horsepower being generated. Recent examples include the used SD40 variants converted in Brazil to meter gauge B+B-B+B.

Now a detour to discuss adhesion. The nominal adhesion on rail is considered to be about 25%. Good dry rail is around 30%, but poor rail conditions can drop adhesion to 20% or lower. Steam locomotives were typically designed at 25% and due to their design their tractive effort was limited. By comparison, a diesel-electric's tractive effort is in direct proportion to its speed, so at low speeds it will continue to pull until it exceeds the available adhesion and slips. Early wheel slip systems sere very primitive and often no more than a buzzer or light to warn the engineer that wheels were slipping. This advanced to a system that would also momentarily drop the electrical excitation and sometimes provide timed sanding. This circuit remained common on most EMD switchers and GP15s. As horsepower per axle increased and adhesion became more critical, advanced wheel slip systems were developed. For instance the EMD IDAC/WS-10 system provides several steps of correction such as timed sanding, momentarily reducing excitation, modulating excitation and finally dropping excitation depending on the severity and duration of the wheel slip. GE and aftermarket systems are similar. Wheelslips are detected by comparing either the electrical balance or speed of traction motors depending on the system. Simultaneous wheel slips of all the axles may not be detectable by the typical wheel slip circuits so an additional circuit to monitor wheel acceleration or overspeed is added.

Once locomotive designs exceeded 500 hp per traction motor, it became common for the available tractive effort to exceed adhesion at low speeds. Back when railroads tended to keep consists of all the same model this wasn't that big of an issue because the engineer could keep a lower throttle position at low speeds. But what if you wanted to run a GP9 and GP40 together at low speed? Builders offered a commonly applied option of power matching where at low speeds the excitation would be limited on new high horsepower per axle designs. Most EMD engines of this era will typically electrically derate to no more than 500 hp per axle at 10 mph or less. So at 10 mph a 3000 hp GP40 will only pull the same as a 2000 hp GP38. Even the SD45 will derate to 3000 hp at 10 mph. About 15 mph they typically can use their full horsepower.

With the introduction of the GP49/GP50/SD50, EMD began to use Super Series wheel slip control. When starting and at very low speeds, these still use a circuit similar to IDAC/WS10 switching to Super Series as speed increases. Instead of trying to eliminate all wheel slip, it was discovered that a certain percentage of "wheel creep" actually provided additional tractive effort. A doppler radar is used to determine true ground speed and then the system allows a certain amount of wheel creep to be able to utilize more of the available horsepower at low speeds. (My limited experience with Super Series does not impress me.) This adhesion theory was also adopted by GE and most new locomotives use some variation of it.

During the last horsepower race, it became obvious that the conventional DC traction motor design had reached its plateau in how much power it could handle. About the same time, advances in static electronic component design made electronic switching practicable. Three phase synchronous AC motors offer advantages in reduced maintenance with a far greater electrical capacity and adhesion control, and they are almost impossible to burn out at low speeds. But this requires an extensive amount of electronic control and an inverter for each axle (or truck depending on design).

BNSF had the idea, and apparently still stands by the concept, that a 4 motor AC unit could replace a six motor DC unit due to the increased adhesion of the AC system. It is their belief that the maintenance savings would pay for itself over the long run. Due to the added weight of a modern locomotive compared to those of 20 years ago - comfort cabs, inverters, high capacity dynamics, numerous modifications for emissions and don't forget 20+ tons of fuel to feed the high horsepower engine between fuel stops - a high horsepower 4 axle AC locomotive is likely impracticable due to weight limitations. GE uses an A1A-A1A design on their C4 design with a weight transfer system that transfers weight of the unpowered axles at low speeds, while the EMD P4 is actually a B1-1B design with no known weight transfer. While it is true that there are modern 4 axle commuter locomotives, these are fat pigs that the builders are being forced to put on a diet to stay within weight limits. Most new commuter locomotives have had steel reduced in size or replaced with high strength steel, aluminum or composites. Fuel capacity is greatly limited due to weight. The new EMD F125 for Metrolink will be using a high rpm Caterpillar engine which is lighter. It may only be a matter of time before the A1A-A1A design returns to passenger service.

If you read the blogs, you will soon find that most BNSF engineers hate the GE C4s. Some of this is still teething problems due to software and other issues, but most of the fault in my opinion is their assignments. BNSF keeps assigning them as a conventional AC unit where they are expected to pull down on their knees, which they simply do not have the weight on drivers to do (regardless of their adhesion system) compared to a six motored AC. I have also read that BNSF assigns power to their trains primarily based on horsepower per ton which can be misleading in low speed drag applications. By contrast I believe UP is still assigning power by tons per equivalent axle rating. With the UP system, a stock DC unit would be counted as 4 or 6 axles, if it is a 50 series or newer DC locomotive it has a higher equivalent axle count and ACs are higher yet. I don't have the current UP info handy, so maybe someone else can post this.

Have some other stuff to get done today so enough for now. Will try to run the numbers again for curves and stiffer grade and post them tonight.