In 1891, engineers built a railway up a Colorado mountain too steep for normal trains; its toothed track still carries passengers to the 14,115-foot summit

Most trains can't climb slopes steeper than about 10 percent. The metal wheels lose their grip and spin against the metal rails. Some areas of Pikes Peak, a granite giant in Colorado's Front Range, are much steeper than that. So, over a hundred years ago, engineers built a different type of railroad, one with a toothed track down the middle. An engineering history record kept for the American Society of Mechanical Engineers states that the line is the highest railroad in the United States and the highest rack railroad in the world. It has carried more than three million passengers to the summit since 1891, without a single passenger death.

The idea came from a miserable mule ride

The railroad began with a difficult mule ride. A businessman, Zalmon Simmons, had come to the Pikes Peak region in 1888 to check out a telegraph insulator he had invented. He got to the top the only way it could be done then: a rough two-day ride on a mule. Simmons, who also founded what became the Simmons Beautyrest Mattress Company, decided the view was worth a more comfortable way up. He financed a company to build a railroad, and construction began in 1889. Italian laborers hacked a pathway from the mountainside, their supplies carried by donkeys.

Solving a slope ordinary trains could not touch

The grade itself was the biggest obstacle. Parts of the route climb at 25 percent, a gradient far beyond the ability of wheels and rails alone. Engineers turned to a system invented by the Swiss engineer Roman Abt. His design put two steel bars with upright teeth between the running rails. Under each engine, a gear wheel was in mesh with these teeth, so that the train could climb the hill tooth by tooth, with the locomotive pushing the passenger car from behind rather than pulling it, a deliberate design choice to prevent runaway cars on the steep grade, instead of relying upon friction alone. The dedication booklet ASME published when it named the line a National Historic Mechanical Engineering Landmark records that the average grade on the line runs over 16 percent and the earliest steam locomotives built had tilted frames so that the boilers remained level even on that climb.

A first run that nearly went wrong

The first passenger train was due to run on June 30, 1891. The first group of dignitaries had left that morning but had to turn back when a rockslide blocked the track. Later that same day, a church choir from Denver boarded a second train and made it all the way to the summit, becoming among the first paying passengers to reach 14,000 feet by rail in the United States. The railroad ran every season for well over a century, until a complete track and infrastructure rebuild closed it from October 2017 to May 2021, its only extended shutdown in 130 years of operation.

A line that kept modernizing

Over the next hundred years, the railroad changed hands and technology many times. In 1925, Spencer Penrose, owner of the famed Broadmoor Hotel, purchased the company. In the late 1930s, steam locomotives began to be replaced by gasoline and diesel equipment. Four of the original steam engines are still around today, displayed at places like the Manitou Springs station and the Colorado Railroad Museum. It was last overhauled in 2021 when the tracks were rebuilt completely, and new Swiss-built trains were introduced.

The mountain behind the railway

Pikes Peak itself has an older story to tell. The mountain is named for explorer Zebulon Pike, who saw it from the plains in November 1806 but turned back before reaching the summit. The Encyclopedia Britannica entry on Pikes Peak states that the mountain is 14,115 feet high and no one actually made it to the top until 1820, when members of an expedition led by Major Stephen Long finally made the trek on foot. It took another seventy years before anyone could make the same climb by train instead of on two legs.

An engineering idea that has not aged

What's remarkable about this railroad is not merely that it was built, but that its basic idea has never had to be replaced. Today's passengers climb aboard a sleek modern train, but the physics that pulls them up the mountain hasn't changed since Roman Abt figured it out generations ago: a toothed rail, a corresponding cogwheel, and too steep a slope for anything else.