CHICAGO (CBS) — A signal problem that caused major delays for Metra and Amtrak trains out of Union Station Thursday was caused by a worker falling on a circuit board, while Amtrak was trying to upgrade computer servers during the height of the morning rush.

In a statement Friday morning, Durbin said Amtrak CEO Richard Anderson acknowledged a “series of errors” caused the glitch that left thousands of Metra and Amtrak riders delayed or stranded on Thursday.

“The most important error they made was to decide to do a server upgrade to their computers during peak hours of service. This should be done in the middle of the night when only a handful of trains are running. Along with that, a worker fell on a circuit board, which turned off the computers and lead to the interruption of service that went on all day long,” Durbin stated.

The senator said Amtrak pledged to make changes when it comes to future computer server upgrades.

Amtrak apologized for the fiasco Friday morning.

“The root cause was human error in the process of deploying a server upgrade in our technology facility that supports our dispatch control system at Chicago Union Station. We failed to provide the service that Amtrak customers, Metra commuters and the general public expect of us,” Anderson said in a statement. “We own the system. We will fix this problem. More importantly, we are taking steps to improve our operations in Chicago, which include appointing a veteran Amtrak executive to make sure we deliver the performance our stakeholders expect of us.”

The signal problem started around 8:30 a.m. Thursday, halting all trains in and out of Union station for nearly 90 minutes before Amtrak started switching track signals manually. Delays continued through the evening rush.

Passengers in Union Station’s north concourse saw delays of two to three hours Thursday evening, after Metra warned everyone to expect delays and find another way home.

Lines inched along, snaking through the Great Hall. On board, it was standing room only on the heavily traveled BNSF line.

Amtrak crews were forced to manually switch the track signals, which is normally done remotely. But there was a breakdown in the communications system.

Some trains unloaded before even reaching Union Station.

About 61,000 passengers ride the six lines that travel through Union Station daily, and about 90 trains leave from late afternoon into the evening commute.