Hartsfield-Jackson International Airport’s Plane Train service was disrupted Monday morning — with a mechanical problem causing congestion at the world’s busiest airport.

The train system disruption caused crowding in the underground tunnels between concourses.

The airport posted on X, formerly known as Twitter, on Monday morning before 10 a.m. that the Plane Train was in “shuttle mode due to a mechanical issue.” Shuttle mode typically means trains operate on limited routes with less frequent service. During busy periods that can mean trains don’t come frequently enough to move the thousands of passengers headed to each concourse.

Some travelers posted on social media that the underground walkways between concourses were packed, and passengers were running late trying to catch their flights. One person called the situation at the airport “pandemonium.”

The airport posted Monday afternoon that the problem was resolved.

“The Plane Train returned to a normal schedule after experiencing a computer issue earlier this morning,” the airport said.

The Plane Trains normally operate roughly every two minutes and carry more than 200,000 passengers a day on a 3-mile loop track, according to Hartsfield-Jackson’s website.

The Plane Train system is undergoing construction and improvements, including a major project to extend the track to increase capacity of the train system. Another project is underway to replace switches for the decades-old people-mover system.