America’s Largest Train Museum Just Saved One Of The Most Important Diesel Locomotives In Modern History

The diesel-electric locomotive has been a mainstay of freight railroads and many passenger railroads for over a century. These locomotives have proven to have a great mix of power, fuel economy, and reliability that helps keep the world turning. One of the greatest advancements in modern diesel-electric locomotives was the AC traction motor. By utilizing alternating current-powered motors, today’s locomotives can have 100 percent greater adhesion than their older DC-powered counterparts while also upping both reliability and fuel economy. The first production American locomotive with AC traction motors, BNSF EMD SD70MAC 9400, was just donated to America’s largest train museum, the Illinois Railway Museum, where it’ll be preserved for generations to come.

This orange, yellow, and black locomotive might not look all that impressive. Most freight locomotives on America’s rails today look just like it! This locomotive was also built at the tail end of 1993, so it’s not even nearly as vintage as most locomotives that you’d find in a train museum. But the Burlington Northern Santa Fe No. 9400 wasn’t just a pioneer; it was the first of its kind. In the few decades after No. 9400 was finished, thousands of diesel-electric locomotives were built with AC traction motors, as that’s now the industry standard. What you’re looking at here is an important turning point in modern train history.

The life of a locomotive is often one without much fanfare. Countless locomotives dutifully serve their railroads, only to see themselves tossed to the scrapyard once their useful lives end. Even rare, historic, and weird locomotives have seen this fate perhaps too many times throughout history. Sadly, just like with classic cars and motorcycles, you cannot save them all, so it’s a real special occasion when a museum is able to rehome a vintage locomotive.

The legendary Illinois Railway Museum (IRM), a non-profit educational organization that’s been preserving train history since 1953, has been on a roll saving special diesels left and right. In just this year alone, IRM has saved a Chicago rail legend with the acquisition of a Metra 614, an EMD F40C. IRM has also picked up Amtrak 231, an F40PHR, and BNSF 6976, an SDP40F. All four of these locomotives represent fascinating points in locomotive history, and landing in the hands of IRM will ensure that railfans can be starstruck by them for decades to come.

Something that I love about IRM is the organization’s “Museum In Motion” concept. It’s not hard to find a museum full of cars, motorcycles, or locomotives that will never run again. IRM is not that. The museum has 5 miles of demonstration track laid down on former Elgin and Belvidere Electric Company right-of-way, over 100 acres of land, over 500 pieces of rail equipment, and even around three dozen motor buses and trolley buses.

What’s great about all of this, aside from the sheer scale of the museum, is that so much of IRM’s equipment is in running condition. The organization, which runs on donations, ticket sales, memberships, and more, has an army of volunteers who do everything from maintaining and driving the trains to performing track work, guiding guests as conductors, and telling the history of each locomotive. IRM is like one huge family located on a mammoth property in Union, Illinois. This is all to say that No. 9400 is going to a good home.

But why am I gushing over this big orange beast in the first place? Well, it has to do with what came before.

AC/DC

Diesel-electric locomotives have been around for over a century, and for most of that time, these locomotives were propelled forward using direct current (DC) traction motors. I’ll explain how this whole thing works.

The core of a diesel-electric locomotive is its diesel engine. These colossal engines are called prime movers, and they do not actually drive the locomotive. Instead, these meaty engines, which usually have horsepower ratings measured in the thousands with displacements above 100 liters, burn diesel fuel to generate mechanical energy. This power is then used to drive a generator (produces direct current) or an alternator (produces alternating current). Here’s where mechanical energy is turned into electrical energy, which is fed into powerful electric motors fitted to the locomotive’s axles, which provide the actual locomotion.

This is why these locomotives are called diesel-electric. However, there are other diesels, which I’ve written about before, including diesel-hydraulic locomotives and diesel-mechanical locomotives. In both of those cases, the big change is in how power is transmitted to the rails.

Nearly all diesel-electric locomotives built before the early 1990s featured DC traction motors. As railroad supplier Republic Locomotive explains, DC systems are quite simple, yet effective. A DC locomotive will have a number of throttle settings with a set power output for each notch. The Railway Technical Website explains how these motors were designed:

The motor consists of two parts, a rotating armature and a fixed field. The fixed field consists of tightly wound coils of wire fitted inside the motor case. The armature is another set of coils wound round a central shaft. It is connected to the field through “brushes” which are spring loaded contacts pressing against an extension of the armature called the commutator. The commutator collects all the terminations of the armature coils and distributes them in a circular pattern to allow the correct sequence of current flow.

As the DC motor starts to turn, the interaction of the magnetic fields inside it causes it to generate a voltage internally. This “back voltage” opposes the applied voltage and the current that flows is governed by the difference between the two. So, as the motor speeds up, the internally generated voltage rises, the effective voltage falls, less current is forced through the motor and thus the torque falls. The motor naturally stops accelerating when the drag of the train matches the torque produced by the motors. To continue accelerating the train, resistors are switched out in steps, each step increasing the effective voltage and thus the current and torque for a little bit longer until the motor catches up. This can be heard and felt in older DC trains as a series of clunks under the floor, each accompanied by a jerk of acceleration as the torque suddenly increases in response to the new surge of current. When no resistor is left in the circuit, the full line voltage is applied directly to the motor. The train’s speed remains constant at the point where the torque of the motor, governed by the effective voltage, equals the drag – sometimes referred to as balancing speed. If the train starts to climb a grade, the speed reduces because drag is greater than torque. But the reduction in speed causes the back voltage to decline and thus the effective voltage rises – until the current forced through the motor produces enough torque to match the new drag.

On an electric train, the driver originally had to control the cutting out of resistance manually but, by the beginning of the First World War in 1914, automatic acceleration was being used on multiple-unit trains. This was achieved by an accelerating relay (often called a “notching relay”) in the motor circuit which monitored the fall of current as each step of resistance was cut out. All the driver had to do was select low, medium or full speed (called “shunt”, “series” and “parallel” from the way the motors were connected in the resistance circuit) and the equipment would do the rest.

These DC motors worked for decades and had been proven to be powerful and reliable enough to help diesel-electric technology surpass the ideas of old, like steam. However, DC motors were not perfect.

DC Quirks

As Republic Locomotive writes, DC motors had funny quirks. One quirk was that if wheel slip occurred during acceleration, friction is reduced enough that a DC motor might speed up and enter a runaway condition, which could result in the motor burning up if the condition isn’t corrected fast enough. The twist here is that since all of a locomotive’s DC motors attempt to operate as one, the cure to wheel slip is to reduce the load to all motors. Because of this, Republic Locomotive notes, a DC locomotive’s maximum adhesion occurs at a level that’s below the theoretical maximum. Newer DC-equipped locomotives use a sort of traction control system to monitor for wheel slip and correct slippage before it gets out of control, allowing a DC locomotive to get closer to its theoretical maximum.

Tractive effort is a locomotive’s pulling force, which, when divided by the locomotive’s weight, equals adhesion. The best way to describe adhesion is that it defines how well a locomotive’s wheels grip the rails.

Another quirk noted by Republic Locomotive is that DC traction motors have an interesting relationship with weight transfer, from the company:

When a locomotive is pulling a load, weight tends to transfer from the front axle to the rear axle of each truck. At maximum tractive effort, the weight on the lead axle may be reduced by about 20%. Since the tractive effort is proportional to the weight on drivers, then in a DC system where the motors are fed from a common source, the tractive effort will be determined by the lightest axle. Thus, in effect, the equivalent locomotive weight is reduced by about 20%.

Because of these three factors, Republic Locomotive says, first-generation DC locomotives like the General Motors Electro-Motive Diesel (EMD) GP9 (above) and the EMD SD40 might have adhesion levels of only 18 percent to 20 percent, while a later DC locomotive like an EMD SD60 or a General Electric Dash 8 might get as high as 27 percent.

AC locomotives would be a major breakthrough as not only did alternating current make locomotives more reliable, but they also dramatically increased adhesion.

A Breakthrough

As Trains.com writes, the locomotive industry was slow to adapt to AC power. AC motors had been adapted by manufacturing industries, mining, mills, and more long before locomotives did. The advantages of AC motors were clear, but locomotives had to wait. Part of the problem was just that solid-state electronics weren’t quite where they needed to be until about the 1970s. The other problem was that, while designing a powerful AC motor might not have been that big of a deal, making motors that could survive railroad duty and could be controlled in a railway setting was the major hurdle.

Trains.com described a major breakthrough that made AC diesel-electric locomotives closer to reality:

This was made possible when gate turn-off thyristor (GTO) technology was developed by GE. GTOs are high-powered semi-conductors that function as an inverter when controlled by a microprocessor, converting DC to AC current. Inverters, however, do not produce a true sinusoidal wave form. Rather, they generate pulse-width modulation that resembles a sinusoidal wave form. The combination of GTOs and microprocessors make variable-speed AC motors practical in robust, high-power applications.

As Railway Age writes, the first diesel-electric locomotive with AC traction motors was Germany’s Henschel-BBC DE2500 (above) of 1971. These locomotives were called the Class 202 and ran on Germany’s state railway, the Deutsche Bundesbahn (DB). Class 202s had 2,500 HP and a frequency-controlled traction drive system with asynchronous traction motors and static converters. The DE2500 was an experimental locomotive, but AC power proved to be so promising that DB put the technology into electric locomotives.

Over in North America, Canada was the first to adopt AC technology. In 1984, the Canadian Pacific Railway launched locomotive No. 4744, which was a 1971 Montreal Locomotive Works M-640 modified to use AC traction motors. However, the real leader in AC technology was the Electro-Motive Diesel of General Motors, which teamed up with supplier Siemens to develop heavy-haul, six-axle, diesel-electric freight locomotives.

From Railway Age:

The Siemens traction motor system used one [GTO] inverter per truck. In 1987, GE Transportation (now Wabtec) followed with an experimental six-axle unit with AC drive and individual regulation of each traction motor with one inverter per axle. In 1989, EMD produced two passenger prototypes, the four-axle F69РНАС, the first U.S. diesel-electric locomotive to use inverters with GTO (gate turn-off) thyristors and evaporative cooling.

Freight locomotive AC traction gained momentum in 1991-1992 with the EMD SD60MAC. These 4,000-hp, six-axle units used Siemens traction motors. Four demonstrators were built. In late 1992, these units demonstrated high adhesion qualities during testing at TTCI (now TTC Operated by ENSCO), establishing that three AC locomotives could replace five SD40-2 DC traction units, which were commonly used on heavy-haul Powder River Basin unit coal trains on the Burlington Northern (now BNSF). After further testing on BN, these locomotives were demonstrated in experimental service on nine North American Class I railroads in the U.S. and Canada.

Making History

AC power had proven to be so great that in 1993, the Burlington Northern Railroad (BN) made the then-unprecedented move of ordering 350 SD70MAC locomotives from EMD. In case you’re curious, yes, that alphabet soup of letters and numbers means something! The EMD SD70 went into production in 1992, originally with DC motors. The SD70MAC is an evolution of the SD70, and its name means “Special Duty, 70 Series, Modified cab, Alternating Current.” The “SD” nomenclature was used by EMD to designate its heavy freight locomotives, and the “Modified cab” there refers to the “North American Safety Cab” design, which gave railroaders operating the train more comfort than other designs.

Anyway, BN’s order of these locomotives was monumental. Until this point, AC locomotives were experimental in America, but here was a major railroad going all-in on the concept.

BN had good reason to. As Republic Locomotive writes, AC locomotives have huge advantages. AC traction works by converting the traction alternator output to DC and then converting it to variable frequency AC, which gets sent to the AC traction motors. AC traction motors operate at approximately the frequency of the current, and the drive system adjusts the frequency so that the motors can run through their full range of RPM. AC locomotives also use robust microprocessor control systems, which, like the computer systems in your car, allow for fine control of practically every parameter of a locomotive.

Remember the quirk with DC motors and weight transfer? AC locomotives can combat this by reducing power output to the less-weighted axle while increasing power to the axle with weight on it. AC locomotives can also control the output of specific motors, allowing the locomotive to operate even closer to its theoretical maximum adhesion. The drive system can also detect wheel slip and kill it faster and more efficiently than a DC locomotive can.

These effects and more allow AC locomotives to have up to 100 percent more adhesion than a DC locomotive of old. In practical terms, this means that a railroad might be able to get away using only one diesel-electric locomotive in an application that might have required two locomotives. AC diesel-electrics are also said to get better fuel economy, too.

As a bonus, AC locomotives have turned out to be reliable monsters, with simpler, lighter motors that don’t burn out during acceleration and require no mechanical contacts like brushes. As Trains.com writes, AC-equipped locomotives even have better dynamic braking than DC locomotives. In dynamic braking, a locomotive’s electric motors work like generators and slow down the train. Per Trains.com, AC locomotives can have dynamic braking so good that they can almost stop on the motors alone.

Production of the EMD SD70MAC commenced toward the end of 1993, and as I explained above, it was a leap forward. Most EMD SD70MACs, like BNSF (now IRMX) No. 9400, were equipped with EMD 16-710 prime movers, which were 186-liter V16 diesels rated at 4,000 HP. These brutes send power to a sextet of GM 1TB2630 motors. These beasts measure 74 feet long, have 175,500 pounds of starting tractive effort, and 137,000 pounds of continuous tractive effort at 12 mph. In case you’re curious, and I know you are, you’re looking at 415,000 pounds.

Burlington Northern, which would later become Burlington Northern Santa Fe (BNSF) after BN’s acquisition of the Atchison, Topeka and Santa Fe Railway, ran No. 9400 for a whopping 25 years, where the locomotive earned its keep. Over that time, AC locomotives like 9400 went from being rare to being commonplace. Today, even major commuter rail operations like Chicagoland’s Metra have AC locomotives on their rosters. Oh, be sure to listen to how the SD70MAC sounds:

Back in 2024, BNSF began preparations to retire No. 9400, and IRM was chosen for its destination. This planning took months, and the locomotive officially joined the IRM fleet in July. Sadly, 9400 will need restoration before it can run on IRM’s demonstration line, but I know IRM’s capable diesel shop will get it running. Either way, IRM has saved another important part of history.

What’s interesting is that, in the days immediately after BNSF 9400 was constructed, it already became a part of IRM history when IRM loaned its iconic Nebraska Zephyr to BN for a formal dedication ceremony that included 9400.

Here’s what IRM says about the occasion:

Following testing during the early 1990s of freight diesels fitted with AC traction motors, Burlington Northern placed the first production order for AC traction motor-equipped diesels with EMD in 1993. The order, for 350 locomotives numbered 9400-9749, was constructed between November 1993 and mid-1996. The 4,000-horsepower locomotives were groundbreaking not only for their AC traction motors but also for their radial trucks, computerized control systems, sound-deadened wide cabs, and “desktop” engineer’s control layout.

BN 9400 was given stainless-steel handrails and participated in a formal dedication ceremony in Fort Worth, Texas, in January 1994. Alongside the new SD70MAC was Chicago Burlington & Quincy E5 9911A “Silver Pilot,” on loan from IRM along with the “Nebraska Zephyr” trainset. Following its debut, 9400 entered daily service with BN and later BNSF, remaining in revenue service for nearly 25 years. In becoming part of the IRM historic collection, BNSF 9400 joins historic EMD products including the first GP-type and first SD-type locomotives ever built as well as CB&Q 9911A, the last E5.

The Museum plans to restore 9400 to operation and eventually repaint it to its original appearance, but donations are needed to cover the costs of storage, mechanical work, and restoration. Click here to support the preservation and restoration of BN 9400.

I got to see No. 9400 during the museum’s 34th Annual Vintage Transport Extravaganza, where I displayed my little Honda Life. It was amazing to stand in the presence of such a giant and important fixture of train history. Ol’ 9400 stood tall and proud. What’s impressive is despite its age, 9400 looks really solid. It’s pretty common for retired locomotives to be peppered with rust holes and ruined paint, but BNSF 9400 is still presentable. It’s going to look so cool in its original BN color scheme.

It’s always interesting to see the starting point of technologies that are so common today, whether they’re planes, trains, or automobiles. AC diesel-electric locomotives are so common today that so many people see them and probably think nothing about them. While BNSF 9400 wasn’t the first in the world, it definitely got the trend kicked off. If you want to see 9400, I highly recommend a visit to the Illinois Railway Museum. I promise you won’t regret it.

Top graphic image: Flickr/Reginald T. McDowell Sr.