After their successful heist of Manhattan Trust, the Cuneos have a problem on their hands - they've stolen too much money to launder through their businesses. The Corleones, however, will have no such problem. Steal the Cuneo stash before it leaves New York.
Reward: +7,500 Respect
Even though you stole most of the money from Manhattan Trust when the Cuneos heisted it, they apparently managed to get away with some of it. Clemenza wants you to steal this money too. Drive to the railyard and approach the Cuneos from the south. When you have them in your sights, go into free-aim mode and shoot the explosive boxes and cars around them. Then shoot whoever's remaining directly. Kill the guy standing on the train car as soon as you can because he's armed with molotovs and he's not afraid to use them.
When everyone's dead, go up the ramp to the northwest of the train car to get inside it and steal the money. There's a film reel in the car as well if you're interested.