German police said Monday they had opened a criminal probe against staff of the state rail operator after passengers were trapped in trains in sauna-like conditions when the air conditioning failed.

"An investigation over bodily harm due to negligence has been opened" against unnamed rail personnel, a spokesman for the federal police force said.

"It will begin with a technical review to determine what caused the malfunction."

The Federal Railway Authority said it would conduct its own investigation of Deutsche Bahn, the state rail operator.

Sweltering temperatures forced the evacuation of around 1,000 passengers from three high-speed trains Saturday after an air-conditioning breakdown made carriages unbearable.

Local media reported that temperatures in one packed train had reached more than 50 degrees Celsius (122 degrees Fahrenheit).

Forty-four of the evacuated passengers needed medical treatment, including nine teenagers who were taken to hospital.

"It was like a sauna," a 16-year-old told the Westfalenblatt newspaper.

Several passengers, fearing they would collapse, had to lie down on the floor of the carriages. The mother of a small boy attempted to smash a window with an emergency hammer to let in fresh air, the newspaper reported.

Rail staff and emergency services workers handed out cold drinks to dehydrated passengers before they were placed on other trains.

Deutsche Bahn has apologised and promised compensation for the affected passengers.

A company spokesman acknowledged that beyond the total malfunction of the air conditioning in the three trains, individual carriages of other trains also suffered a breakdown in their cooling systems.

Staff had now received orders in instances of a total malfunction to stop at the next possible station.

Transport Minister Peter Ramsauer phoned the company's chief Ruediger Grube and called for improvements in future models of the ICE high-speed train, a ministry spokeswoman said.

Deutsche Bahn has faced mounting criticism over repeated technical problems on passenger trains, compounded by a recent scandal over internal spying on company staff.

Critics say a cost-cutting drive to prepare for an initial public offering has compromised service. The privatisation was postponed indefinitely after the financial crisis hit.