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Based on my experience with industrial controls, embedded computing, data logging and modern sensors, I think it would very feasible to duplicate the capabilities of a dynamometer car.

It could possibly be done with just a properly instrumented coupler shank.

Heck, just use the whole car. It'll be a lot more burly, but a lot longer, so you'll still have measurable stretch and compress forces.

Just a matter of building a pilot beam that runs from one end of the car to the other (it can be supported by hangers), anchored on one end, test instrument on the other. The carbody stretches and compresses, the pilot beam does not, compare them, easy peasy. Just like the little indicating beam on a torque wrench, or the sense wires off an ammeter shunt.

Oh, you could build it, and depending on what the car was meant for and what instrumentation you were working with, it wouldn't have to cost that much.

That brings to mind, most dynamometer cars didn't measure just drawbar pull, they also measured speed and a number of other things. Certainly in locomotive testing, you measured more than that. In steam, that could include steam pressure and temperature, exhaust temperature, water consumption, fuel consumption, and throttle and cutoff positions. No doubt there would be diesel counterparts to be measured as well.

And then would follow the work of assembling the data and interpreting it, though modern computers would make pretty short work of that--after you had written the program first!!

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They already exist. Amtrak, NS, CSX, BNSF, UP, the FRA, etc. have cars that duplicate their functions. They just don't call them "dynamometer cars" because they also measure so much more. (A staggering amount more, as I found on my rides.)

Further, as Kelly indicated, the drawbar pull measurement function is already incorporated into modern locomotives.

This is kind of like saying "why don't we create a line of pocket texting and video chat devices?" and everyone will answer "Why?? I already have my iPhone/smartphone!"

Incidentally, "a PC"?? Amtrak's track geometry cars feature solid WALLS of high-speed, high-powered computer arrays to power and record that vast array of detectors, gauges, video, etc.

Outside of the previously mentioned alternatives available to mainline railways, it's fairly simple to create an effective dynamometer car.

Early in his career, L D Porta needed to measure the performance of the highly modified meter gauge 4-8-0 he built from an earlier 4-6-2. IIRC, he added a calibrated speedometer to an existing passenger car or caboose. He then had a section of metal of specific dimensions fabricated to put between the coupler on this car and the coupler on the locomotive. A strain gage was placed on the part and the readout for the strain gauge was placed in the car. The strain on the part could be directly correlated to the locomotive's drawbar pull.

For a fraction of the cost of a "proper" dynamometer car, it was an effective solution.

How do modern diesel locos measure drawbar pull? I can't believe they have a strain gauge on the drawbar... that would be another maintenance item of no real value.

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The problem with this approach is maintaining calibration of the strain gauge. The old "Dyna Couplers" did this but were found to be unreliable.

The NS Dynamometer Car that was used to measure the performance of N&W 611 a few years ago was equipped with an instrumented drawbar that was connected to a laptop computers. Video screens in the "business" end of the car provide real time display of the parameters being measured (speed and drawbar pull) and the uncorrected horsepower.

The one shortcoming was the lack of GPS for location. Each Milepost had to be manually entered (most miles were less than 5,280 feet in length). This was critical for the horsepower correction as grade and curvature are inputs to the correction.

I can easily believe that even a Challenger could not pull against modern high-capacity AC dynamic brakes on say a SD 70, 80, or 90 AC. One of the most impressive things about the GE AC6000's is that they can pretty much bring a heavy train to a complete stop, even on a downgrade. Also, steam locomotives have a power curve, with it's maximum potential hp well above 0 mph in most cases, while the highest potential tractive effort for a D-E loco is from a standing start.

The rule of thumb for dynamic brakes that I was taught is that older, standard dynamic brakes have their strongest retarding effect around 26 mph, and it falls off below that, and quit working around 6 mph, and newer "extended range" dynamic brakes are good to about 2 mph. AC dynamic brakes are a truly awesome tool for an engineer to use, and a large part of the reason that many (most?) railroads currently train freight engineers to not use the air any more than absolutely necessary, pretty much just to finish stopping and hold a stopped train.

The first part is true, Kelly. The second part never happened. Dynamic brakes have to have SOME movement to function at all. Standing still, they do nothing.

This leads to something I first thought was incredible, but after thinking, makes sense. One of the Operating Practices gurus at work explained to me that with AC traction , you can actually put the locomotive into reverse, and work the throttle against the load, creating dynamic braking without pumping current into the grids. If you did this with DC, you would probably have all the bells ringing in the electrical locker. This is the only way they could have done what is alleged.

Steve

IF they have a 'holding' function on AC locomotives, they are not dynamic brakes. I can't see any good reason an AC loco could not have a holding function, but it would be an active control providing 'counter-torque'. Traditional dynamic brakes provide resistance to movement but at zero RPM there would be zero brakes.

I have switched a GE 25 ton into 'reverse' to stop it (no brakes), but never at any speed above a crawl (1/3 walking speed?). Never any more throttle then the bare minimum to get the contactor to close. Based on how 'harsh' that seems, I could see it being quite destructive at any speed above a walk. I don't know if there is protection built into the larger units but otherwise I wouldn't even bet the equipment would survive long enough to get you to a stop...