What do I have to do to unlock the "Train Heist" level?

When opening up locked boxes inside armored cars in the new DLC heists, there is a chance that one of the boxes will contain intel. Picking up this intel item will unlock a subsequent "Train Heist" level after you complete your current heist. However, picking up the intel does not unlock the "Train Heist" level permanently.

The intel, when it drops, seems to be placed in the locked box closest to the back doors of the armored transport, so opening the trucks up with C4 will reduce your chances of finding intel.

Well since the Spring Break update you can buy this contact from Bain in the contract broker section.