Interrogate Train Robbers

There are two train robbers marked in the map but you only need to interrogate one of them. Both of them are located in the trainyard so take your pick. There's a sentry that's usually with him as well as a handful of guards so it's okay to go out loud as long as you attack the informant first to disable him. Deal with the sentry next then take out the rest. A well-placed grenade or molotov should be enough to disable the informant. In case he is spooked, the yellow car nearby is his escape vehicle so you can also intercept him there.

Steal Santini’s Money Stash

This will become available after interrogating one of the two train robbers. Head to the marked location. The cash is located on the second floor and fortunately, there’s a backdoor with stairs leading to the second floor. Sneak in the area and stealth kill any guards. Break the lock in the door to open it then sneak inside. Kill the lone lookout there then steal the cash. You don’t need to kill everyone but if you want to inflict more damage, you can destroy the weapon crates below.