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.
