What exactly is Hell Rush? I played a bunch of Monster train at release and don’t recall that. I do have the DLC, but haven’t played since I got it.
It’s the default online multiplayer mode, under the Challenges menu.
You’re playing against real people, high score wins. Or being the last survivor.
It forces you to get better because everything’s timed.
Edit: The DLC adds a pretty neat mechanic in the shard faction, a new final boss, and a bunch of new cards iirc.