Ryder has heard about an ammunition train and wants it, and so do the Vagos, who are already robbing it when Carl and Ryder arrive. The Ballas turn up behind them, so the mission becomes a three-way fight over a line of freight cars with Carl defending the thing he came to steal.
Once the train is clear the crates have to be got out, and the police arrive with them. The Pay 'n' Spray is written into the objectives, which by now is the game's shorthand for expecting a wanted level rather than hoping to avoid one.