LOUISVILLE, Ky. -- State investigators are trying to find out what caused Monday's train derailment at the Louisville Zoo.

More than 20 people were transported to local hospitals, including two children. One child, a 21-month-old, is listed in serious condition.

Investigators spent Tuesday taking pictures, examining the scene and conducting interviews.

The Department of Agriculture said the operator of the train was 18 years old. That is the minimum age for an operator in the state of Kentucky.

The train began operating in 2000. State officials said it passed all recent inspections. The last inspection was January 22.

“State law in 2008 also requires the ride operator to inspect and test the ride everyday before its open to the public," Sloan said.

More than 24 hours after the crash, the train is still on its side, detached from its wheels. Its wires and mechanics are all exposed as inspectors try and figure out how and why it jumped the tracks.

"I can tell you the Department of Inspectors will look at everything the electronics and the mechanics the pneumatics to determine if it was in working order," said Ted Sloan, of the Kentucky Department of Agriculture.

Those who were riding the train have their own ideas about what happened.

"It started going a little fast through the tunnel and went around some curves and kept gaining speed and they said the breaks went out and then it just feel over," victim Darren Bamforth said.

The spokesman said the inspectors will depend on facts to determine the official cause.

"We are going to do a thorough investigation. No matter how much time it takes to get to the bottom of what happened and why it happened," Sloan said.

Zoo officials said the train is given a pre-trip inspection daily by the driver which includes a 35-point checklist.

The train is also inspected weekly by the zoo's maintenance staff.