Answer: BNSF Railway uses two models of six-axle locomotives with only four powered axles: GE Transportation’s (now Wabtec Freight) ES44C4 and Progress Rail’s SD70ACe-P4. The two builders have chosen different arrangements for power: GE uses an A1A-A1A arrangement while EMD uses a B1-1B setup.
Translated from the Association of American Railroads wheel arrangement system, letters are powered axles in a row, where A is one axle, B is two, C is three, and so on. Numbers are unpowered idler axles in a row. An A1A truck has an idler axle in between two powered axles while a B1-1B truck has the two axles closest to the fuel tank unpowered.
Both models of locomotives use alternating current for traction. They aim to give railroads the increased reliability of AC traction at the lower power and cost of a direct current locomotive which has all six axles powered.
Having all six axles powered is mostly useful at low speeds. Once at speed, horsepower sets the pace. At low speeds, the ES44C4 automatically shifts some weight off the unpowered axle using a compressed air cylinder, giving the powered axles more weight to increase wheel-rail adhesion. — Tyler Trahan
Hey, Al Diecenso.
The purpose of the A1A-A1A on the ES44C4 and B1-1B on the SD70AceP4 is to help spread the weight of the locomotive over more axles to minimize rail wear and keep the axles from breaking. For example: an ES44C4 weighs in at 416,000 lbs. With SIX total axles that means each axle carries 69,333 lbs. each. Remember that the maximum allowed axle weight is 72,000 lbs. like on a heavy type. If the weight of the C4 is spread over just 4 axles, like a B-B arrangement, then you have an astonishing 104,000 lbs. per axle pressing down. That can break axles and severely damage the rail. So, when not pulling hard, the unpowered axle is lowered to spread carry its share of locomotive weight.
Also, the theory behind GE’s design of the ES44C4 truck with the air-powered lift crank was to allow the highest amount of adhesion and weight transfer to the powered axles to allow for maximum tractive effort output on FOUR powered axles. The axle life mechanism is air powered to allow the middle unpowered axle to lift up just enough to accomplish that task (when working properly). It also allows a less complex lift system to be in place, instead of working with an electrical lift mechanism that would be prone to numerous electrical glitches and faults.
Lastly, a B-B can handle turns better than a “fixed geometry” truck (unlike a self-steering/radial), but it cannot transfer the weight across more axles, therefore the maxi,um weight is limited to what would be 72,000 lbs. of max weight per axle.
(If this doesn’t fully answer your question, let me know where you need additional information.)
This does not answer the question of why have the un-powered axle at all, instead of just using a B-B configuration for full time max tractive effort. What beneficial effect does taking weight OFF of the powered axles via air pressure or other means provide? In addition, a four axle truck inherently tracks better than a six axle truck (unless it’s a “radial”, or self-steering type).