Two of Honolulu’s driverless trains knocked together in what’s known as a “hard coupling” during what rail officials said was a routine maintenance exercise Monday at the Pearl City rail yard.

A hard coupling happens when two trains connect to each other while moving too fast, according to a mainland-based transit monitoring group.

During the Pearl City incident earlier this week, the two trains were traveling 3 to 5 mph on a yard storage track when they connected “head-to-head” via an electrical coupling unit, according to a statement Wednesday from Hitachi Rail Honolulu.

Hitachi has a $918 million contract with the city to run the future rail transit service, and trial running of the system is currently underway. The company’s statement was provided via the Honolulu Authority for Rapid Transportation, which oversees rail construction.

The hard-coupling occurred as Hitachi conducted routine train “repositioning” around 11 p.m. Monday in the yard, according to its statement.

The company did not specify whether the two trains were supposed to connect during that repositioning. However, it described the Monday incident as “out of the ordinary.” No staff or personnel were injured, according to its statement. It did not specify whether the trains sustained any damage.

It’s not clear how the incident impacts trial running, if at all.

“It is important to note that the trains remain in the testing and commissioning stage at this time,” Hitachi’s statement read. The company and HART “remain committed to examining and testing equipment to ensure functionality aligned to design standards,” it added.