During the day, the subway stations we are familiar with are always busy and noisy, but every 12 pm, in the quiet of night, in the dark and narrow subway tunnels, there is a group of people who claim to be "underground workers" who have just started their work, and Master Yuan is one of them. Master Yuan's work is somewhat similar to that of a doctor, but their pulse is not directed towards patients in the hospital, but towards the silent steel rails in the subway. Every day, when a subway train carries thousands of passengers between cities, due to the rolling of the train and the change of Thermal expansion of metal, the rails in the subway often have various diseases.

When a train falls off the track, it means accidents such as derailment and overturning occur. To ensure safety, Master Yuan and his team undergo a "physical examination" of the steel rails every day. The patient's condition varies in severity, and the steel rails are the same. Their examination must be meticulous and meticulous. This machine is called an ultrasonic rail flaw detector, which can detect whether there are cracks inside the rail. Since 1987, Master Yuan has been responsible for the inspection of Line 2 since he started working on rail flaw detection. He has been walking on this circular line for 26 years, and the most memorable thing for Master Yuan is that he discovered more than 200 large and small problems with the steel rail.

That night, when Master Yuan and his team finished their work, it was already over two o'clock in the morning. We habitually said to Master Yuan, 'You can rest tomorrow.'. Master Yuan said, 'It's not tomorrow, it's today.'. At this moment, we realized that a new day had already begun.