Documentaries about migrants have become a thriving subgenre, thanks to an abundance of subjects crossing the globe. Rarely are they presented with the kind intimacy of Raúl Pastrana’s film, which presents the trail from south Mexico to the US in vivid detail, filled with the weary but amused resignation of displaced people. Many migrant documentaries are about the final journey; this one is about waiting, unsure if there will ever be a final journey to a better life.

Our main character is Gustavo, a Nicaraguan who has been shot by Mexican police while attempting to get closer to the border. Migrants such as Gustavo play a cat-and-mouse game with the authorities, whose presence as they ride the top of trains in full military fatigues and balaclavas is a frightening warning in the film. If the troops are deployed elsewhere, the migrants are the ones riding in dense formation on the roofs. This back and forth of trains is a backbone of the film, and Pastrana shows us as much of the dust and hot metal as possible in some spectacular frames.

This game has rules of engagement, one of which is that the police should not use excessive gunfire on migrants, or at least not be caught doing so – and Gustavo’s case has had significant attention in the media, which gives him a welcome leg-up on his fellow migrants. We return again and again to Gustavo’s waiting game, and also visit an array of other Central Americans, mostly Hondurans, hanging out in camps, in forests and by the tracks, waiting for an opportunity. With both the US and Mexican authorities cracking down, their chances are bleak, and the film is peppered with talk of those who’ve disappeared and died.

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There’s a solidarity and pleasure in the migrant camps, even though these friendships may be temporary. Coffee is made in an ingenious way and shared, and delicious-looking Mexican food is whipped up on the cheap for a communal meal. There are some imaginative skills on show – fixing shoes on the fly, making sculptures, making the best of what’s lying around. There are some frighteningly young-looking migrants, who do at least appear to be looked after. We don’t often see migrants doing the best they can like this in stasis, at least not in this part of the world. There are some stunning set-piece scenes, such as the endless train that could liberate them crossing above their heads every day as they lounge on chairs in a communal yard, and plentiful use of eerie silhouettes of figures on freight trains in the dark.

Less successful is the other half of the film, featuring Jason, an American anthropologist tracking the remains of migrants in the borderlands. He’s a likable and determined figure, and there’s poignancy in his treks through the desert finding ephemera such as sun-faded backpacks as he seeks to document and name the many unidentified or lost people. However good his intentions, I found it difficult to care about his academic approach and his neatly folded bags of lost possessions, compared to the real struggles of people attempting to join him in the United States. Jason can pop over the border any time to do his research, while the equivalent journey is of monumental difficulty to those in the other half of the film. While this irony is clearly intentional, it sits oddly.

This quibble aside, Pastrana has made a caring documentary with humanity. We receive the message strongly that these are ordinary people expected to do extraordinary things to live like the rest of us. Gustavo’s destiny becomes perilous when he is no longer the centre of media attention, no longer regarded as exceptional; we don’t end the film with much more hope than when we started. This is skilful film-making, not to present the migrants as heroes, but just as people with mundane plans and dreams like the rest of us.

• Border South will be shown at Sheffield Doc/Fest on 8 and 10 June.