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Railroads are an artifact of steam technology, and steam technology was and remains a transitional technology.

A steam engine is an example of a heat engine. In a heat engine, one adds energy to a working fluid, then exploits some characteristic of the fluid to extract the energy as useful work. In almost all cases the working fluid is either a gas or a substance that becomes gaseous when energy is added, and the characteristic exploited is the tendency of a gas to expand when heated. Add energy to the gas — heat it — but trap it and prevent it from expanding. Deliver it to a mechanism where its expansion is allowed provided it moves something mechanical. As it expands, the gas becomes cooler — the energy put into it has been extracted. Voilá: Heat from the energy source has been turned into mechanical motion, which can (for example) turn a shaft and move a vehicle.

Soon after the possibility became apparent, a Frenchman name of Carnót worked out Da Rulez for heat engines in general, which is that the efficiency of the process is absolutely determined by the ratio of the temperature of the gas when it enters the mechanical part and the temperature when it leaves. Aha! you say. The machine can be infinitely efficient if the temperature on exit is zero! Well, yes, but part of that definition is absolute temperature. Zero Celsius freezes water, but it’s possible to get lots colder than that; there is still energy in the fluid at zero Celsius. That leads to the concept of absolute temperature. When the absolute temperature is zero, the fluid contains no energy, and it’s the temperature in absolute units that is used to calculate efficiency in the Carnót cycle.

Steam engines have two working fluids: the combustion products from burning the fuel must be transferred to the water to make steam, and the steam is then delivered to the part that extracts the energy as mechanical motion. That means there are two steps in which efficiency must be calculated; that means a steam engine is inefficient, and that, in turn, means that it isn’t very powerful for a given size and cost. Railroads are a way to make a big, expensive, not-very-powerful prime mover useful. A train, with hard wheels running on a hard rail, has the lowest friction (and therefore loss) of any land transportation device. If friction is low, a relatively weak but persistent prime mover can eventually get the load up to speed, because it is working against inertia and not friction, and that inertia persists, so the device keeps moving. The same is true — even more so, in fact — for boats if the speed is low.

If you have a more-efficient engine, you can squander part of that efficiency on overcoming friction and still be more efficient overall. One way to do that is to only use one working fluid, and the obvious way to do that is to do away with the intermediate step and use the combustion products directly. Early engineers couldn’t do that because the temperatures are very high, and they didn’t have materials that could stand up to them or ways to overcome the problem. Gas turbines use materials that can stand the heat — the exhaust from a turbine (a jet engine, for instance) is still hot, but the ratio of absolute temperatures between the flames entering the turbine and the exhaust is high, so the machine is relatively efficient. Reciprocating engines, like what’s in your car, do it a different way. The temperature at the instant combustion occurs is very high; the resulting hot gases push the piston down, and the gases are exhausted at the end of the stroke at a relatively low temperature. It’s efficient, and the fact that combustion only occurs occasionally means that the mass of the engine can average out the temperature, so you can use fairly ordinary materials to make it.

The greater efficiency of internal combustion engines means they can be smaller and cheaper, and that some of that efficiency can be used to overcome the friction of a simpler (and therefore less expensive) travelway. A railroad is horrendously expensive — it must be level and straight and made of materials that are hard to handle, because it’s built to reduce rolling friction and mash hills down to the point that a weak prime mover can handle the load. It’s also terrifically complex, because allowing rails to cross while maintaining the necessary straightness and smoothness is a hard problem, and because it’s a restricted resource, making scheduling cumbersome and expensive. Roads are much cheaper: the materials are easier to get, they can cross with no trouble, you can add or subtract vehicles without complex scheduling, and it isn’t necessary to go to such great lengths to move mountains around to keep them level, because the engines that run on them are relatively more powerful.

That, in turn, gives flexibility. Roads are cheap enough that they can go almost anywhere, even right up to your very doorstep — imagine the expense of putting a train track up to where people could board the train a few steps from the door! They don’t have the crossing problem, because the designers need not go to elaborate lengths to keep crossings from losing power. We accept a loss of efficiency in the system as a whole in order to achieve flexibility, which makes the overall transportation system more efficient. That’s why people everywhere go for the automobile like a pack of rats as soon as they become available.

The only reason railroads persist at all is that no matter what you do a weak engine is cheaper than a powerful one, and the rails were already in place, legacy of the steam era. Train engines are remarkably powerful in automobile terms, up to some tens of thousands of horsepower compared to the pitiful few hundreds in the most powerful car — but the car only pushes a couple of tons around, where the train engine moves as many as a hundred rail cars, each over a hundred tons of vehicle and cargo. The rail system can be maintained relatively cheaply, and the engines are relatively cheap, so trains continue to be useful — but if we were starting from scratch, with no transportation system at all, nobody would ever build a railroad! It’s just too inefficient from an overall transportation-system perspective to carry the stuff to a train depot, load it on the train, let the train move it awhile, then offload it and transfer it somehow to the people who need the stuff. Trains are useful if the volume and mass of the stuff is great enough to make it practical to build a “door-to-door” system — multiple tons of coal from mine to power plant, for instance — and not otherwise. European passenger rail still sort of works on the margin because Europeans live closer together, so it isn’t all that far from home to the train depot; the transshipment problem is less. North America is much more thinly populated, so it is (on the average) a much longer way from home or factory to the rail facility. Even in Europe, though, it makes no sense to go five kilometers to the railroad, ride the train ten kilometers, then go five kilometers to the destination; the automobile covers the ten kilometers directly and at less expense.

High speed rail is even more absurd. The cost of building the rails goes by the cube of the speed desired: a railroad built for 60 MPH is eight times as expensive (well, not quite) as one for 30. That means that there can’t be many of them, which makes the transshipment problem thornier, more cumbersome, and therefore more expensive. There’s also a subtle difference with profound implications: in a road system with automobiles, the users buy the rolling stock. That gets too complicated to add to an essay on transportation efficiency, but it’s a very important factor.

Put it all together, and trains make no sense except as a legacy system still in use for a limited purpose. Why, then, do people call for more of them?

Building a railroad is a big job, using lots of workers, materials, and machines. Trains are big and complex, and there are always people who find big, complex systems fascinating. The efficiency of the railroad itself can blind people to the transshipment, crossing, and scheduling problems. Add it up, and you have something damned near irresistable to politicians — the project is obvious to everybody, so the politician can be seen as “doing something”, and by skilfully exploiting the romance (big, cool stuff) and handwaving the system efficiency losses (onloading and offloading take place out of sight, thus simply don’t happen for lots of people) the pol can make the project look worthwhile. For a statist politician, one who believes in (or cynically exploits  the perceived value of) centralization, the train is even more attractive because it’s One Big Central Facility Everyone Must Use. The depots are also choke points, making it easier to check up on what and who gets transported.

So in the end, building railroads is a political stunt not based on real economics. Keep that in mind when people start floating the notion.

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January 2011