Even though several years have passed, Zhu Yan still feels a thrill when he remembers the moments during the process they developed the Fuxing (Rejuvenation) bullet trains to be used on the Beijing-Zhangjiakou High-Speed Railway - a route that linked the two host cities of the 2022 Winter Olympic Games.

Zhu, from Chinese railroad rolling stock manufacturer CRRC Changchun Railway Vehicles Co Ltd, a Jilin province-based subsidiary of CRRC, became deputy chief designer of the trains when the company set up the design team in 2017.

In April 2016, construction began on the world's first ballast track, which is designed for speeds up to 350 kilometers per hour. Its route went from Beijing to Zhangjiakou and began to operate in the end of 2019.

It reduced travel time between the two cities from three hours to about one.

"In the beginning, we were all clear that there were lots of technical problems waiting for us to overcome," said Zhu. "We started a three-month enclosed office time with working from 8 am to 8 pm every day."

"We made 42 sets of plans for the locomotive and 5,289 various structural drawings," he said. "However, the biggest difficulty was the complex geophysical conditions along the rail line."

There are 64 Bridges along the rail line, measuring 66 kilometers, and 10 tunnels, measuring 49 kilometers, accounting 66 percent of the whole line.

"It has quite steep slopes, which cause rapid elevation changes," said Zhu. "It costs only 30 seconds to pass from an altitude of 1,700 meters to 1,000, which brought great challenges to the air conditioning pressure system and control system of the train."