Which uses less energy and emits less pollution: a train, a bus, or a car? Advocates of rail transportation rely on the public’s willingness to take for granted the assumption that trains — whether light rail, subways, or high-speed intercity rail — are the most energy-efficient and cleanest forms of transportation. But there is plenty of evidence that this is far from true.

Rail advocates often reason like this: the average car has 1.1 people in it. Compare the BTUs or carbon emissions per passenger mile with those from a full train, and the train wins hands down.

The problem with such hypothetical examples is that the numbers are always wrong. As a recent study from the University of California (Davis) notes, the load factors are critical.

The average commuter car has 1.1 people, but even during rush hour most of the vehicles on the road are not transporting commuters. When counting all trips, the average is 1.6, and a little higher (1.7) for light trucks (pick ups, full-sized vans, and SUVs).

On the other hand, the trains are rarely full, yet they operate all day long (while your car runs only when it has someone in it who wants to go somewhere). According to the National Transit Database, in 2007 the average American subway car had 25 people in it (against a theoretical capacity of 150); the average light-rail car had 24 people (capacity 170); the average commuter-rail car had 37 people (capacity 165); and the average bus had 11 (capacity 64). In other words, our transit systems operate at about one-sixth of capacity. Even an SUV averaging 1.7 people does better than that.

When Amtrak compares its fuel economy with automobiles (see p. 19), it relies on Department of Energy data that presumes 1.6 people per car (see tables 2.13 for cars and 2.14 for Amtrak). But another Department of Energy report points out that cars in intercity travel tend to be more fully loaded — the average turns out to be 2.4 people.

“Intercity auto trips tend to [have] higher-than-average vehicle occupancy rates,” says the DOE. “On average, they are as energy-efficient as rail intercity trips.” Moreover, the report adds, “if passenger rail competes for modal share by moving to high speed service, its energy efficiency should be reduced somewhat — making overall energy savings even more problematic.”

Projections that high-speed rail will be energy-efficient assume high load factors (in the linked case, 70 percent). But with some of the routes in the Obama high-speed rail plan terminating in such relatively small cities as Eugene, Oregon; Mobile, Alabama; and Portland, Maine, load factors will often be much lower.

Even if a particular rail proposal did save a little energy in year-to-year operations, studies show that the energy cost of constructing rail lines dwarfs any annual savings. The environmental impact statement for a Portland, Oregon light-rail line found it would take 171 years of annual energy savings to repay the energy cost of construction (they built it anyway).

Public transit buses tend to be some the least energy-efficient vehicles around because agencies tend to buy really big buses (why not? The feds pay for them), and they run around empty much of the time. But private intercity buses are some of the most energy efficient vehicles because the private operators have an incentive to fill them up. A study commissioned by the American Bus Association found that intercity buses use little more than a third as much energy per passenger mile as Amtrak. (The source may seem self-serving, but DOE data estimate intercity buses are even more efficient than that–compare table 2.12 with intercity bus passenger miles in this table).

When it comes to energy consumption per passenger mile, the real waste is generated by public transit agencies and Amtrak. Instead of trying to fill seats, they are politically driven to provide service to all taxpayers, regardless of population density or demand. One of Amtrak’s unheralded high-speed (110-mph) rail lines is between Chicago and Detroit, but it carries so few people that Amtrak loses $84 per passenger (compared with an average of $37 for other short-distance corridors).

Meanwhile, transit agencies build light-rail lines to wealthy suburbs with three cars in every garage. With capacities of more than 170, the average light-rail car in Baltimore and Denver carries less than 15 people, while San Jose’s carries 16. For that we need to spend $40 million a mile on track and $3 million per railcar (vs. $300,000 for a bus)?

If we really wanted to save energy, we would privatize transit, privatize Amtrak, and sell highways to private entrepreneurs who would have an incentive to reduce the congestion that wastes nearly 3 billion gallons of fuel each year (p. 1). But of course, the real goal of the rail people is not to save energy but to reshape American lifestyles. They just can’t stand to see people enjoying the freedom of being able to go where they want, when they want to get there.