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Collision Course
A primer on how train wrecks of the past paved the way for safe data transfer today.
Published March 1996
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BUY A REPRINTA primer on how the train wrecks of yesteryear paved the way for technology that keeps data transfer safe today
On July 17, 1856, a hot, humid day in southeastern Pennsylvania, two passenger trains traveling in opposite directions on a single track crashed head-on just outside Fort Washington.
It was a tragedy of monstrous proportions. The train bound west, a special-excursion train, was crammed full of boisterous schoolchildren en route from Philadelphia; the one headed east, a local train, carried sleepy commuters on their way to work. All told, 60 people died, and 60 more were injured. That reverberations from the disaster would be heard more than 100 years later seemed as likely as snow in summer.
Yet those echoes are everywhere -- in the way telephone calls are forwarded from one party to the next, to the way E-mail flies safely from computer to computer. Traffic, after all, is traffic -- whether it's made up of steam engines on tracks or high-speed data streams in fiber-optic cable. What caused the two trains to taste metal could just as easily make a computer network crash: faulty protocol design. Systems engineers have taken to heart the lessons learned from the crash in Pennsylvania in 1856 and other railway accidents, and have developed sophisticated software to make sure that E-mail and other traveling data arrive intact.
In the early days of railroads, train engineers had tried to do the same for passengers. They developed sets of rules to govern traffic on the rails ("protocols," in computer parlance), to make sure that trains traveling in opposite directions over a single track could do so with impunity.
In 1856 one such protocol was the North Pennsylvania Railroad's rule for handling train delays: Any train awaiting the passing of a train traveling in the opposite direction had to wait at least an additional 15 minutes if the oncoming train failed to appear. The conductor of a delayed train had to make sure to move his train onto a siding if he could not make it to the regular meeting point within that 15-minute interval.
On that fateful day in July, the trains' respective conductors had tried to follow the rule to the letter. The special-excursion train was scheduled to arrive in Fort Washington at 6:00 a.m. The local train was scheduled to leave the Fort Washington station at 6:15 a.m. However, the special-excursion train was held up; it did not make it to Fort Washington by 6:00 a.m. So the delay rule had to be applied. But there was a fatal flaw to the rule: it did not specify whether the 15-minute grace period started at the time the first train (the special-excursion train, in this case) was scheduled to arrive or at the time the second train (the commuter) was scheduled to depart. As it happened, the conductor of the local train assumed it was the former, and the conductor of the special assumed it was the latter. The two trains collided shortly after 6:15 a.m.
