Why Do Escalator Steps Have Teeth?

On October 23rd, 2018, thousands of football fans were making their way to the game in Rome. Excitement for the game was high, and the fans began to chant and sing. At 7:03 p.m., around 50 people were riding the long escalator down to the platform. But within 30 seconds, the crowd had swelled to nearly double that.

Everything seemed fine, but inside the escalator was a problem. The weight of the passengers was bearing down on the steps and the load on the main motor was increasing. To try to slow the descent, the motor applied a counter torque. But as the force continued to increase, the stairs began to move faster.

By 7:04 p.m., the crowd had tripled. The motor finally reached its limit, and under the massive strain, the drum began to slip. With the motor losing control, the escalator triggered its second line of defense. A safety relay tripped immediately, cutting power to the motor.

The main brake clamped down on the metal drum to stop the descent of the stairs, but it failed. The friction on the main drum wasn't enough to stop the motor from spinning, and the stairs continued to accelerate. Assensing the motor had lost control of the steps, the escalator engaged the last line of defense. In the event of an emergency, an auxiliary brake is designed to bypass the motor entirely and directly lock the drive shaft.

Under normal circumstances, the chance that all three safety measures fail at the same time is vanishingly small. But these weren't normal circumstances. At 7:05 p.m., the third and final safety system failed, and the stairs began to plummet. Fans were flung forward and started streaming down the escalator.

Some leapt over the central barrier in desperation, while others were swept into a crushing pileup. At the bottom, the landing became a dangerous choke point. Under the pressure, the steps twisted and buckled into jagged metal, leaving 24 people injured. Something like this shouldn't have been possible, and experts at the time knew that something had gone wrong, and they started to suspect foul play.

In the aftermath, Rome's transit agency sealed off the accident site and closed the Republica station for several months. The authorities ordered both a technical and a criminal investigation, and the mayor even publicly vowed to discover the cause of the accident. So, investigators began dismantling the wreck, ...