Catalyst mission
When you reach the train, do not bother getting out of your car. Drive around the pinned down Groves and run into the Vagos. Drive into the train tunnel to turn around if needed. After all the ones on the ground are killed, exit the car and shoot the one on the train. Then, get back into your car and run over the arriving Ballas. If your car begins smoking, park it next to the Ballas car to create an explosion that should kill them all. Then, get on the train as instructed. Quickly toss the ammo crates to Ryder. There is an unlimited number of them onboard so you do not have to waste time getting a perfect target. You may want to change the "Invert Look Controls" setting to make this less confusing. Remember to change the setting back after all the crates are tossed.