A small train carrying visitors to the Louisville Zoo fell off the tracks Monday, sending 22 people to hospitals including one child who was critically injured, officials said.

The train of open-air cars is pulled by a small engine and circles the zoo along a two-mile (three-kilometer) track. It was carrying about 30 passengers when three cars and the engine fell off the rails near the gorilla exhibit. A person briefly trapped was able to be freed, zoo spokeswoman Kara Bussabarger said.

Seventeen children were taken to Kosair Children's Hospital for treatment, including one in critical condition and another in serious, said spokesman Brian Rublein. Five adults were taken to University of Louisville Hospital, and spokesman David McArthur said all were in fair or better condition and that one might be admitted.

Those injured were from 2 months old to senior citizens, he said.

Rublein said besides the two children admitted to Kosair, three others were still being evaluated, with the rest treated and released.

The derailment happened about 1 1/2 hours before the zoo's 6 p.m. closing, Bussabarger said. The train wasn't in a public area but behind the gorilla exhibit, she said.

Zoo officials planned to reopen as scheduled Tuesday, but the ride will be closed until further notice. Bussabarger said three Kentucky Department of Agriculture ride inspectors are investigating. The cause was not yet known, Bussabarger said.

The train ride opened 40 years ago, Bussabarger said in a statement. The one that derailed was bought in 2000, holds 40 to 50 passengers and averages 12 miles (19 kilometers) per hour, she said.