Why Railroads Make Clean-Energy Sense

With all the hoopla around alternative-propulsion vehicles (e.g., electric cars and hybrids), not too much gets into the mass media about the efficiency of rail transport. Those of us who've lived in the Far West don't need to be reminded of railroad transportation. Running individual truckloads of goods along highways one-by-one seems kinda silly next to mile-long freight trains.

BNSF Railway Company recently announced two free tools for shippers and carriers to be able to directly compare the cost and transit times of intermodal service to the highway alternative. The company says that shipping by intermodal rather than solely by highway provides important environmental, safety and security benefits.

In the late 1949s, when then Gen. Dwight Eisenhower became impressed with the German Autobahn system, rail cars had to be individually loaded and unloaded at each end of any transportation run. So, a shipment of barbell weights cast in China would have been loaded on a ship, sailed across the Pacific Ocean, unloaded in, say, San Pedro, Calif., then unpacked from the freighter and repacked into boxcars. After making the rail trip to, say, Erie, Penn., they'd have been unpacked from the train, and repacked into freight trucks for delivery to wherever they were finally to go. All that packing and unpacking took a lot of time and labor.

A few years later, when elected President of the United States, Eisenhower took the opportunity to reproduce the Autobahn system on a grand scale in the Interstate Highway system.

Figure 1: Movement of a truck through the atmosphere builds high air pressure ahead and pulls low pressure behind. Both effects create retarding forces on the truck. This effect is called induced drag. (Click to expand)

Since then, however, we've developed the intermodal transportation system. With intermodal, everything destined for that Erie location from that China starting point would be packed in ISO shipping containers, which are exactly the size and shape of the boxy trailers that long-haul trucks pull, sans the wheels. At the Chinese port, these containers would be tightly packed into cavernous holds of dry-shipping freighters headed for the U.S. port. Once there, the containers would be lifted out of the holds, and stacked two-to-three high on railroad flatcars made for the purpose. The flatcars would be made up into trains for the transcontinental passage. In Erie, each container would be lifted from the rail cars onto an individual truck for final delivery. That makes the system time and labor efficient.

I won't go into the safety and security benefits, as they are not basic technology issues. The environmental aspects, however, stem from basic physics and engineering. Specifically, high-speed rail transport can be (should be) more fuel efficient than highway transport.

There are two main forces that cause vehicles to burn fuel at high speeds: aerodynamic drag and rolling friction. I'll start with rolling friction.

The main cause of rolling friction is deformation of the wheels as they generate reaction forces to support the vehicle's gross weight. These deformations convert kinetic energy of the rolling wheel to heat. The more deformation and the faster the wheel rotates, the greater the heat. That is why truck tires are so large in diameter (fewer revolutions per mile) and why they run at very high pressures (less deformation). Rail cars, on the other hand, have steel tires, rather than pneumatically supported rubber tires, so they hardly deform at all, even when carrying the enormous gross weight of a fully loaded rail car. Thus, rolling friction per unit weight in railroad transport is a fraction of that for highway transport.

Aerodynamic drag arises from the need to elbow air out from in front of the vehicle, then suck it back to fill the hole in the atmosphere after it passes. Figure 1 shows how high pressure builds up in front of a highway freight truck, and low pressure forms behind it. These high and low pressure regions create forces that hold the truck back - aerodynamic drag.

Figure 2: Running two trucks in tandem dangerously close together neutralizes the low pressure region behind the first truck and the high pressure region in front of the second, reducing the aerodynamic drag by nearly half. (Click to expand)

Drag forces increase as the square of the vehicle's speed, so they rapidly become the dominant energy-loss mechanism.

Truckers soon learned that the best way to reduce aerodynamic drag is to run trucks nose-to-tail close enough so the high-pressure zone in front of the following truck overlaps the low-pressure zone behind the truck ahead. As Figure 2 shows, the two pressure zones cancel each other out, effectively cutting the net aerodynamic drag in half. This so-called drafting technique has been used for decades by truck "convoys" to lower operating expenses by saving fuel. The more trucks in the convoy, the more fuel saved.

Figure 3: Because they are physically coupled together, railroad cars can run safely with very little spacing between them, providing huge aerodynamic drag advantages. This drafting phenomenon effectively neutralizes induced drag for all but the lead and last cars. It does not, however, reduce viscous drag caused by sliding of air past the cars tops and sides. (Click to expand)

The same effect improves aerodynamic efficiency of railroad transport, except that the trains are very much longer and the cars can be safely run very much closer.

These two effects boosting overland transport efficiency makes maximizing use of rail transport good energy policy. What we, as ordinary citizens, can do is raise the volume of voices calling for increased use of rail transport as part of energy policy. Since the same phenomena apply to passenger transport compared to individual cars, we should also clamor for upgrading commuter rail as an alternative to commuting via cars.