SAN ANTONIO -- A Union Pacific Railroad train derailed early Monday, sending box cars and thousands of gallons of diesel fuel into a shallow portion of the San Antonio River.

The derailment occurred at 4 a.m. at Roosevelt Avenue and South St. Mary's St. after a Union Pacific Railroad train headed to Mexico from Houston struck the last car of a Union Pacific Railroad train headed to Illinois from Mexico, said Union Pacific Railroad Spokesman Mark Davis. The trains were on separate tracks when the accident happened.

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Watch Aerial View of Derailment

The engineer and conductor of the train that derailed jumped off their locomotives before the derailment and suffered minor injuries. A homeless man was also slightly injured and was hospitalized at Christus Santa Rosa Hospital downtown.

Two locomotives and 12 cars from one of the trains derailed, Davis said. The locomotives and five cars ended up in the San Antonio River, spilling 5,600 diesel gallons of diesel into the river. No cars from the other train jumped the tracks.

Crews set up a dam in an effort to stop diesel fuel from flowing down river and an absorbent was also being utilized.

Davis said 500 feet of track will be replaced.

He added it will take several hours to clean up the wreckage.

The cause of the derailment is under investigation.