Deadlands is an RPG originally published in 1993. It's been through a few iterations and currently is under Savage Worlds rules, I think.
This very old book is from the original run.
What happens if the supernatural comes back to the world during the American Civil War? Well you get gunslingers dealing with hucksters, people who make deals with demons to sling magic and Blessed running around trying to save the world with Mad scientists makings thing better (or far worse) while all dealing with blood thirsty tumbleweeds (tumblebleeds) and the dealing with the Reckoners, unknown True Evils.
The original system was its own creation, using a deck of cards including 2 jokers and multiple types of dice, it was an exciting system. Eventually the old system was replaced a free times and it's currently published under Savage Rules.
I can only talk on the original system, quite liking the use of decks of cards for initiative and other uses. Combat could be quite slow, but it is a very flavorful system.
This is a bad photo, but it's the book from our shelf!
I ran a dnd campaign that was wild west themed, and I used a few plotlines from this source. The Lost Angels storyline is pretty good