In Shanghai, subway maintenance workers are a devoted crew, but largely unseen and unsung. The city's Longyang Road Station is now hosting a collage of more than 600 photos taken by Metro employees that document the network's expansion over the last 20 years.

660 photos chronicle two decades of subway history?

Shi Weiwei is a maintenance engineer for the maglev train that connects Shanghai Pudong International Airport and the outskirts of central Pudong. His daily job is to maintain the traction system and dispatch repair teams when the system detects a breakdown. Also an avid photographer, Shi enjoys documenting the life of those who work on the front line of metro maintenance.

For example, Shi took this photograph last August of his coworkers adjusting the track beams of the maglev train at 1a.m. He says these moments show the passion Metro workers have for their work.

"I used to work on the front line myself. So I can totally understand how physically tiring their job is. They work when most of us are already asleep, which is why the public rarely notices them. But it's their hard work that ensures the normal operation of the maglev trains," said Shi Weiwei, maintenance engineer, SH Maglev Transportation Development Co.

Shi's piece is just one of 660 photographs assembled in a collage exhibited at Longyang station, with the oldest taken back in 1993. The collage is named "Impression Metro," a collection of observations and memories about the Shanghai subway network by its 30,000 employees.

Xin Yaojin, who has worked for the Shentong Group for almost 30 years, is one of them, contributing nearly 20 photographs. He is a co-creator of the project. A photograph, taken right before the opening of Line 16, is his favorite.

"I remember it was almost the end of 2014, right before the opening of a new line. The workers were laying cables that day. To be honest, I was moved by how involved they were in what they were doing. I thought I should document that moment," he said.

And the photos are encouraging others to take photos.

"This is the first time I saw something like this. I thought it was very creative, so I took a picture," said Wang Jie, an interviewee.

From a distance, this collage looks just like a pair of eyes. The creator says it not only symbolizes the history and development, but also the future of the Shanghai Metro network.

Shanghai's subway network now comprises more than 600 kilometers and 366 stations. By 2020, four new lines will be added, which will add another 200 kilometers.