Updated 05/20/15 – 4:32 p.m.

CHICAGO (CBS) — Service on Metra’s Electric Line was back to normal Wednesday morning, after an overnight derailment caused major delays at the start of the morning rush.

Two of the four cars of the last northbound train of the night slipped off the rails around 1 a.m. near 11th Street, according to Metra spokesman Tom Miller. Five passengers were on board at the time, but were not injured. The last train of the night from downtown Chicago to south suburban University Park was canceled after the accident.

There are a total of three tracks in the site where the train derailed, and the incident caused a bottleneck at the start of the morning rush on Wednesday, with all trains forced to use a single track.

Metra says they believe the a slight dip on the tracks led to the wheels on the opposite side coming off the track and causing the derailment, based on a preliminary investigation. They do not know how the dip got there and are investigating, but foul play is not suspected.

Initially, Metra was warning Electric Line commuters delays on morning trains could be up to 1 hour and 40 minutes, but by about 6 a.m., a second track had been opened, significantly reducing delays, according to Metra spokesman Michael Gillis. As of 6 a.m., Metra was estimating delays on the Electric Line would average 15 minutes.

Trains on the South Shore Line between downtown Chicago and northwest Indiana also were delayed 15 to 20 minutes during the morning rush, due to the Metra derailment.

As of 9 a.m., no delays were reported on the Electric Line or the South Shore Line.

The cause of the derailment was under investigation.