For this mission you need to meet with President Manuela in Refugio Umbra to the northeast. Grapple to Teo’s helicopter to fly over to the train, then grapple onto the train and ready your heavy weaponry for the upcoming engagement. As the train leaves the tunnel a helicopter attacks. Take it down, then watch for the next wave of attackers and deal with them accordingly.
Head toward the back of the train as you finish off the attacking enemies, then shoot the coupler near the back of the train to prevent an impending crash. When the next train approaches, destroy it then head back to the front of your train and grapple onto the next inbound train. Move up to the front car and plant explosives to derail the train and complete the mission.