Investigators are looking into whether a switching error led to a low-speed collision between a Metra train and an Amtrak train at Union Station that injured at least 12 people, none of them seriously, officials said.

One railroad source said both trains were moving when at least two cars of the Burlington Northern express from Aurora struck the Amtrak train bound for Carbondale around 8:30 a.m. The source said the investigation will focus on possible errors related to the switching of the trains. The collision occurred at the south end of the station, where there are many track switches in the yard, the source said.

Metra had hoped for a normal evening rush hour. But Federal Railroad Administration investigators didn't finish signal testing until around 4 p.m., adding to delays getting trains in and out of the station and requiring Metra to reroute trains to other tracks, a Metra spokeswoman said.

Riders should still expect some delays this evening although "we're back in business" and all tracks were now clear, Metra spokeswoman Meg Reile said.

"People shouldn’t just walk where they normally do — they need to pay attention to announcements and the boards," Reile said.

Amtrak owns Union Station and is responsible for the maintenance and condition of all tracks, switches and other equipment at the station. Amtrak's control center at the station directs the movement of all trains, including Metra, there and operates the switches.

Amtrak communicates with Metra locomotive engineers and conductors, similar to how air traffic controllers direct pilots. Once off the union station property, trains are controlled by the railroad that owns the tracks. That would be Burlington Northern in the case of the Metra train involved in today's collision.

The accident happened about 8:15 a.m. between tracks 2 and 4. The station was shut down until 10 a.m., when trains resumed arrivals and departures. Some passengers on affected trains were bused or found other ways downtown. In all, 12 people were taken to hospitals, but none received life-threatening injuries, a Fire Department spokesman said.

An inbound express Metra Burlington Northern train No. 1242 from Aurora coming into Union Station on Track 2 hit the engine of the Amtrak train, said passenger Kirk Musselman. He estimated the Metra train was going about 5 mph.

"There were blown windows, blown glass" on his car, which was the fourth or fifth car back on the train, he said. He said the corner of his car and perhaps one or two others hit the other train on a track alongside.

"It was pretty abrupt," he said. "As we turned, the cars were sticking out and they struck the engine of the other train."

Musselman said he suffered a knee injury and some other passengers were hurt.

Dave Dahler, 64, was headed to his job at the Aon Corp. from Naperville when "all of a sudden there was a jarring thud, like we hit something, then we went a few more feet and we felt the same thing," then the train stopped. "The conductor ran back through each of the cars."

All the conductor said was "we derailed," he said, then there was an announcement that everyone should leave the train.

"We walked back south out of Union Station until we were out. There were police and fire and Amtrak police everywhere. They also were asking for a doctor because one of the ladies went into labor. She was on a car toward the front."

When he first saw her, "she was walking hand in hand, with four or five firemen around her. Her husband said she was pregnant with twins, due to be delivered in three weeks."

Dahler said it sounded like she was going to be okay, though they were trying to contact her obstetrician.

Dahler said he did not see what had happened, but a friend said it appeared one of the Metra cars was slightly off the tracks.

"My buddy said he saw one of the cars was ajar, a little bit off the track," Dahler said. "I think that everybody was just a little bit shocked and surprised, but it didn’t seem like anyone was badly injured. We saw one woman on a stretcher, the pregnant lady on a chair, another woman on a chair."

Another passenger, Matt Sink, said he was in one of the last cars of the Metra train when there was a "real big jolt."

"It's kind of like a movie," he toldWGN Radio, with helicopters overhead and fire trucks surrounding the station and firefighters going through the cars. "It was just an ordinary Friday, I was reading the paper, then a real big jolt."

But Sink said he didn't realize his train had struck another until his wife texted him. "I have not seen any serious injuries," he said.

Metra spokesman Miller said at least two cars of the Burlington Northern train were damaged. It was due into the station at 8:15 a.m.

One Burlington Northern train behind the train involved in the collision was stopped for hours at Western Avenue. At the conductor’s direction, almost everyone got off to search for other ways into downtown.

Several dozen beachgoers clad in bathsuits and flip-flips headed north on Western for a Blue Line stop several blocks away to take the CTA the rest of the way downtown.

Claire Madden, 18, of LaGrange was on her way to the Ohio Street beach with a group of friends.

 “For us, it’s kind of a pain, but we’ll get there eventually,” she said, expressing regret about the people hurt in the collision.

(Passengers can send their pictures and accounts of the accident to tips@tribune.com, or you can call 312/222-5555.)