I just remembered this one today. Fandango...it is about 5 guys who are drafted into the Vietnam war and who go on one last fling/road trip in a '59 Cadillac. Out of gas in the desert, they end up near the ATSF railway. When the train passes one guy lasso's the caboose with some steel cable, gets into the car and WHAM! The entire front body section is ripped off and goes "skiing" down the track.
Great shots of late 60s/early 70s Sante Fe geeps (GP30 or 35) and red offset cupola caboose....great soundtrack, too.