Cary Fukunawa's Sin Nombre is pacy, moving and beautifully shot but, taken as a whole, is less satisfactory than Maria Full of Grace, largely because its docu and drama elements haven't gelled together quite so well.
The factual underpinnings of the story derive from Fukunawa's field trips on top of freight trains traversing Mexico and the homework he subsequently did on the Salvatrucha subculture.
These provided him with his two main protagonists and their convergent plot strands of trek and chase: Sayra, the young Hondureña following her father and uncle on their New Jersey-bound odyssey, and Willy/El Casper, the fine-mannered fugitive marero,
Edgar Flores and Paulina Gaitán are both gratifying screen presences, but Fukunawa's skill with character depth and dialogue falls short of what Joshua Marston achieved in creating Maria for Catalina Sandino Moreno. So, while the drama of their journey never ceases to be gripping, at the same time it never ceases to feel essentially rather contrived. There's ultimately more truth in the background than in the foreground of this tale.
BTW, if you've never been to Tapachula this movie may well scare you off for good, but it's actually a pleasant little town!