Now, with the expansion of high-speed train networks, these old-fashioned people carriers have mostly been phased out, with some still in operation to connect largely to remote or mountainous areas.
"But I believe the green slow trains have become an integral part of the collective memory of a generation, and a few still act as an important means to transport villagers in poverty-stricken areas," says Ren.
She adds that the slow trains, which are much cheaper and have nearer stations, are more affordable and effective especially in rural areas.
In the first episode of the series, travel writer Qi Dong alongside old-fashioned train enthusiast Ma Hao take a ride on the Chengdu-Kunming railway, a major line of around 1,000 kilometers between the two cities－the respective capitals of the provinces of Sichuan and Yunnan.
In the course of their trip the duo encounter a series of unlikely passengers: A local herder who is traveling with 58 sheep, an ethnic Yi bride wearing gold jewelry on her way to meet her future husband in a remote village and a teenager going to pick up her younger brother from his primary school at the next station.
The ticket costs the herder 7.5 yuan ($1.09), and he doesn't need to pay for the sheep that he has brought along on the train to be sold at a market.
As for the siblings, they spend an hour climbing a mountain to get home.
"It's a train of stories of poverty and a hard life, but it's also a train full of hope," says Qi in the documentary.
