It’s an annual tradition created by accelerated modernization, an expanding economy and the world’s largest populace: Each year, 130 million Chinese peasants, displaced from their villages to work in urban factories, crowd train stations to return home for the lunar New Year.

Lixin Fan’s insightful new documentary Last Train Home, a featured selection at this year's San Francisco Film Festival, captures the phenomenon at its alarmingly frantic peak, concentrating on a single family, the Zhangs, divided by harsh economic realities and struggling to cope with the strain.

Fan, whose directorial debut stays impressively focused despite the vastness of his subject, also distinguishes himself here as the cinematographer behind the film's sweeping portrait of the world's largest annual migration. It is a breathtaking spectacle, created by bustling crowds so densely populated that Times Square on New Year's Eve seems roomy by comparison, yet amid the chaos Fan highlights factory workers Zhang Changhua and Chen Suqin, a married couple returning home to visit the children they barely know.

The sacrifices Changhua and his wife have made to provide better lives for Qin, their teenage daughter, and younger son Yang, are significant: For 16 years, they have toiled away at their sewing machines, manufacting garments for export in a city, Guangzhou, more than a thousand miles removed from their rural home in the Sichuan province. Yet their absence, however selflessly motivated, has taken its toll.

Qin, who splits her time between school and subsistence farmwork, resents the parents who reluctantly left her to be raised by her grandmother. She craves the modern conveniences of the city, where so many of her friends have already migrated to chase their dreams of comfortable living. And she defiantly turns a deaf ear to Chen Suqin, who stresses the importance of good grades and a well-rounded education.

The Zhangs are a family estranged, as much by the living conditions dictated by China's economy but also by the changing values of an ever-evolving society. Home, filmed over the course of three years, lingers long enough to witness Qin's rejection of life on the farm – to her mother's dismay, she leaves school to become a factory hand. Whether she has improved her lot or merely resigned herself to the false promise of a dead-end job remains in question.

That is the tragedy of Fan's deeply affecting documentary, in which the Zhangs and so many others like them, who pack the trains each year for an abbreviated glimpse of their homes and loved ones, share little but a common bond of futility.

Last Train Home is now playing at the Lumiere Theatre. For tickets and showtimes, click here.