The Great rail wars are indeed somewhat outdated, however still available trough rpgnow/drivetrough rpg. (sides to a coin right?) But the rules updates to showdown are mainly to make them more omni-genre where as great rail wars was already intended for The weird west (that’s what they cal their deadlands setting themselves) there are a few expansions for it, and you should take into account the different editions. Perhaps most importantly the miniatures are still being produced. (and some of those mini’s are award wining!)
You should also look into their multiplayer campaigns for it! Some real good mix of RPG and war gaming there, with loads of interesting events happening mid scenario etz.