Zhou Yimin, a deputy chief engineer at the ministry, told the financial magazine Caijing that the trains' maximum operating speed should be 300 kph (185 mph) rather than the originally targeted 350 kph (216 mph).
Railways Minister Liu Zhijun, who was dismissed this spring amid an investigation into unspecified alleged corruption charges, insisted on claiming the world's best technology for all aspects of the showcase high-speed rail program, Zhou told Caijing.
Liu demanded higher speeds for the 215 billion yuan ($32.5 billion) Beijing-Shanghai railway to 380 kph (235 mph) despite knowing that the contracts for the foreign-designed cars being used called for a maximum of 300 kph, he said.
"What he wanted was buying a train with a top speed designed at 300 kph but making it run at a speed of 350 kph," Zhou said.
Liu was the public face of the high-speed rail program. No details have been released about the allegations against him, but news reports say accusations include kickbacks, bribes, illegal contracts and sexual liaisons.
As the ruling Communist Party approaches the 90th anniversary of its founding, on July 1, it is seeking to burnish its image for graft-busting.
Liu's case shows that "any corrupt member, whoever he is, will be held accountable," the Communist Party's vice chairman for discipline, Wu Yuliang, told reporters in Beijing on Wednesday.
Trial operations of the 1,318-kilometer (813 mile) Beijing-Shanghai railway began May 11. Its formal inauguration is expected next week.
The Railways Ministry recently specified that the trains will operate at a maximum 300 kph instead of 350 kph, for the sake of efficiency. Ministry officials insist that decision was not based on safety considerations.
"He retired too early to know what is going on," Wang Bin, a ministry spokesman, said of Zhou's comments.
While Zhou did not report severe safety problems or accidents, his criticism is unusually outspoken given the railway's status as a showcase project on a par with Beijing's space program. China is aggressively marketing its expertise and competing for overseas projects, sometimes to the chagrin of foreign partners.
Although they balk at speaking out publicly out of fears of jeopardizing their own business prospects, some working in the industry have criticized China's claims of leading the world in the high-speed rail sector, saying that its achievements are mainly based on foreign knowhow.
Zhou concurred with that assessment, saying China has made significant progress but still lacks core research and development.
The rush to be first has prompted officials to push ahead despite safety glitches, he said.
"In fact, glitches are common in several of our high-speed lines. Some may seem small but actually aren't, and all of them are being kept secret," Caijing quoted him as saying.
Zhou said a high-speed line between Beijing and the northeastern city of Shenyang has been halted several times due to malfunctions, and an engine casing had flown off a train on another line.
Despite questions over whether the costly lines are best suited to China's needs, Beijing aims to have 8,000 miles (13,000 kilometers) of high-speed rail in place by the year's end and twice that length by 2020.