An Expo Line train being tested in Santa Monica derailed Thursday after hitting a truck at the intersection of Colorado Avenue and 7th Street.

Trains have been running periodically on the tracks for several months as part of the testing program on the local portions of the light-rail track, which connects to downtown Los Angeles. Thursday’s crash was the first major incident involving a train in Santa Monica.

The accident happened at about noon as the truck driver attempted to make an illegal left turn from westbound Colorado Avenue onto southbound 7th Street, according to Santa Monica police. The westbound train hit the cab of the truck, which was carrying construction materials. Left turns are prohibited along most of Colorado Avenue adjacent to the tracks and the truck’s turn was in violation of posted signs.

The train came off its tracks in the accident but remained upright, and officials were working Thursday afternoon to get the train back onto the tracks.

A driver and two other officials were aboard the train at the time of the incident, police said. One was complaining of minor injuries. The truck driver was transported to a hospital with minor injuries, Lt. Richard Lewis said.

Santa Monica police are looking into the accident and the investigation is ongoing. Officials from the Santa Monica Fire Department, Los Angeles County Sheriff’s Department and Metro also responded to the scene.

Colorado Avenue was temporarily closed between Lincoln Boulevard and 6th Street, and surrounding roads were heavily congested Thursday afternoon. Officials urged motorists to avoid the area while the train was nudged back onto the tracks and driven out of the area.

Expo trains have been undergoing tests along the extended track from Culver City to Santa Monica in recent months. The extended line is expected to be open to passengers by mid-2016, although it’s unclear whether Thursday’s accident will impact the timeline.

More Expo Line testing is scheduled for Dec. 14-18, according to a City of Santa Monica update.

The 15.2-mile, $2.5-billion Expo project includes three stations in Santa Monica, including the western terminus at Colorado Avenue and 4th Street. Much of the funding is coming through Measure R, a sales tax initiative that Los Angeles County voters approved in 2008.

City Manager Rick Cole said recently that Santa Monica police are working with other public safety agencies to prepare for Expo’s arrival. He said there are numerous potential violations associated with the forthcoming transit option, including walking on the track, driving on the track and making illegal turns along track-adjacent roads.

“We have a lot of education and some enforcement to do,” he said. “Obviously our emphasis is going to be on education, but in order to protect people’s lives when the trains begin running seriously, we’re going to have to be very vigilant to enforce the law to protect people’s safety both on the train and on the street.”

Santa Monica Fire has been actively preparing for Expo’s arrival. SMFD has been participating in tabletop exercises with other agencies like Metro, SMPD and Public works. Deputy Chief Tom Clemo said firefighters have worked through 13 different kinds of Expo incident including train. vs. vehicle accidents, train vs. pedestrian, terrorist attacks, overcrowding at train stations, fires on a train or at a station and several law enforcement scenarios.

Two Santa Monica trucks have been outfitted with specialized equipment capable of lifting a derailed train and Clemo said one truck is located on each side of the track.

He said the timing of SMFD’s training was fortuitous as firefighters were actively training on Expo incidents Monday, Tuesday and Wednesday prior to the Thursday accident. He said this week’s training focused on how to control a train with activities like stabilizing the trains, removing the power sources and accessing control panels.

The combined training showed results Thursday with all the relevant agencies working together at the scene.

“With fire and police, there’s a critical integration with Metro that went very well today,” he said.

Earlier Thursday, the Metro Blue Line was suspended after a body was found under the train in the 4100 block of South Long Beach Avenue, south of downtown Los Angeles.