A miniature train that derailed and injured 22 people at the Louisville Zoo underwent daily inspections and passed a state examination in January, safety officials said Tuesday.

The zoo reopened at its normal time Tuesday morning while state investigators in charge of amusement rides worked to figure out why the train derailed Monday on ground-level tracks in a bend behind the gorilla exhibit. The train ride, which loops two miles around the zoo, remained closed.

One child, a 21-month-old girl, was critically injured when three open-air cars and the engine fell off the tracks. She was upgraded to serious condition on Tuesday, said Brian Rublein, a spokesman for Kosair Children's Hospital.

A 2-month-old boy remained hospitalized in good condition Tuesday evening, Rublein said.

Ted Sloan, a spokesman for the Kentucky Agriculture Department, told WLKY-TV the driver of the train was 18 years old, which is the minimum age for ride operators under a new state law governing amusement rides.

Two recent zoo visitors said they believe the train travels too fast.

Adam Galasso, 25, told The Courier-Journal that he, his wife and their 2-year-old rode the train Sunday for the first time. He said his wife almost lost her balance on one of the curves and came close to falling out of the train's open sides because the train was going so fast.

Zoo member Rod Mitchell said his 4-year-old daughter loves the zoo and the train ride. But he told the newspaper that he's noticed the train going at speeds he felt were too fast several times during the past few months and has complained.

"This could have definitely been avoided," Mitchell said. "It scared me to death."

The zoo said Monday that the train travels at an average speed of 12 miles per hour.

Inspectors from the Agriculture Department returned to the zoo Tuesday to continue investigating, Sloan said.

Zoo officials allowed the media to take photos and video about 20 feet from the derailment site Tuesday, but officials did not answer any questions.

Sloan said the ride operator must test and inspect the ride daily and that an inspection took place on Monday before the zoo opened for the day. The ride also passed an inspection on Jan. 22, before opening for the season and had received no prior complaints, Sloan said.

Bussabarger said in an e-mail Tuesday night that drivers use a 35-point pre-trip checklist to inspect the train daily, including a check of items such as the wheels, brake pads, cables between cars, brake controls and any fluid leak. Each week, the zoo's maintenance staff goes over an 18-point checklist on the train, she said.

Maintenance staffers also inspect the track at least once a month, and under state law, the Agriculture Department must inspect annually, with random inspections authorized, Bussabarger said. She said the zoo uses an internal certification process for train drivers.

The zoo said the attraction has been around for about 40 years, but the train that derailed was bought in 2000.

Officials said there were about 30 people on board when the train derailed.