A killer train with spider-legs is on the loose, and you have to stop it with your own train, That's about it.
The whole thing takes about four hours or so to complete, including side-missions. The game is structured a bit like an RPG, with side quests from NPCs who reward you with new weapons or scrap metal (which is needed for improving stats for armour, speed, and damage, as well as train health You can slo collect scrap by exploring the open game world on foot (you can hop on/off the train at any time to explore).
Worth a look for a short but relatively high quality experience for an indie title.