Idaho Transfer (movie, 1973)

A crew of young researchers escape into the future to avoid the shutdown of their project. They find that some type of 'eco-crisis' disaster has de-populated the area around their lab (in rural Idaho) and, by implication, the nation or maybe the world. Stranded in the barren future, in their travels they encounter an abandoned freight train possibly full of corpses (maybe, it isn't very clear). One of them travels further into the future and meets a family in a futuristic automobile, implying that that humankind has recovered from the disaster (or have they?)