Day 2 is another trading mission. It also serves as punishment if you did Day 1 bad, as you get caught after the trade and need to fight some cops. Head into the train and answer the phone.
He'll ask you to place the paintings on the table in the train, so get to moving them from the van and throwing them onto it.
You'll get called back quickly afterwards, and everyone will be asked to enter the train. Get all players into the train and it'll seal shut.
A chopper will quickly fly over the train and a hatch will open. Grab the paintings and throw them up to the guy. For each painting, he'll throw down a bag of money for your keeping.
The train doors will open after you've thrown all the paintings up, and you can get out. Carry the money bags down a quick walk to the van. This is the part where you could get jumped by the cops, and if you do, you need to fight your way to the van with a timer.