BEIJING—China will begin forcing its growing fleet of high-speed trains to operate at slower speeds, the country's railways chief said in an interview with state-run media, in the latest sign of trouble for the country's most vaunted transportation project.

Sheng Guangzu, head of China's Ministry of Railways, said in an interview with the Communist Party's People's Daily newspaper published Wednesday that the decision will make tickets more affordable and improve energy efficiency on the country's high-speed railways.

Mr. Sheng, who took over the Ministry of Railways in February after his predecessor resigned amid a corruption investigation, said trains in China needed to serve all parts of society.

"China is vast and there are regional differences in economic and social development, so railway construction norms can't be uniform," Mr. Sheng said in the interview. He didn't specify how much ticket prices might be reduced.