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HOME > PAST ISSUE > March-April 2007 > Article Detail

COMPUTING SCIENCE

Trains of Thought

Computing with locomotives and box cars takes a one-track mind

Brian Hayes

Guided by an unseen hand, a grimy railroad tank car negotiates a series of switch points in the tracks, veering right, then right again, then left. Next comes a lime-green box car, which makes two lefts. I observe these events from the control tower of a railroad facility called a hump yard, where freight cars sort themselves into trains bound for various destinations. It is an eerie scene. The cars glide silently downhill through the maze of tracks, seeming to steer themselves, as if each car knows just where it wants to go. This is an illusion; a computer two floors below me is making all the decisions, setting the switches a moment before each car arrives. But I can't shake the impression that the hump yard itself is a kind of computer—that the railroad cars are executing some secret algorithm.

Rail carsClick to Enlarge Image

It's not such a far-fetched notion. In 1994 Adam Chalcraft and Michael Greene, who were then at the University of Cambridge, and later Maurice Margenstern of the University of Metz, designed railroad layouts that simulate the operation of a computer. The machine is programmed by setting switch points in a specific initial pattern; then a locomotive running over the tracks resets some of the switches as it passes; the result of the computation is read from the final configuration of the switches.

These constructions are wonderfully ingenious, although admittedly they have little to do with the day-to-day running of real railroads. Even at a more practical level, though, brawny steel rails and brainy silicon chips have surprisingly rich connections. The work of the hump yard is a case in point. Algorithms for sorting are a specialty of computer science, but railroads were sorting freight cars decades before the first electronic computer was built. Methods invented by rail workers have served as metaphor and inspiration for the development of algorithms and data structures in computer science; conversely, the theoretical analysis of algorithms has suggested ways for railroads to improve their operations.




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