Deadlands: Hell on Earth (1998)

From the back of the book:

The year is 2094, but the future is not our own. The Last War ended 13 years ago when the supernatural doomsday bombs fell, killing billions and terrorforming the world into a vast Deadland, allowing the mysterious and powerful Rockoners to manifest in the flesh. These unholy beings rampaged across the Wasted West, decimating the survivors of the Apocalypse, then mysteriously vanishing across the Mississippi to parts unknown.
Since then, humanity has struggled to rebuild. The cities shudder beneath still-howling storms, leaving only the wastes to those few who remain. Deadlands: Hell on earth is completely compatible with its companion game, the award winning Deadlands: The Weird West. This is not the end of the story, but a twisted start to a tale unlike any you've ever played before! It's once again a time for heroes. Players take on the roles of radslinging Doomsayers, vengeful Law Dogs, brain-burning sykers, scavenger junkers, and righteous Templars -- or simply gun-toting survivors. Only by warring against the darkness all around them can the heroes hope to end this Hell on Earth - and they'd better hurry. Some say the Reckoners are about to return. And soon.