Uncle’s missions are a bit more 'energetic' than either your wife’s or your son’s, so make sure you always have your favourite weapon at the ready. Meet up with Uncle and then ride your horse to get the cattle out of their pen.
Your cattle herding skills should be pretty slick by now, so keep them all in check as you both make your way across the fields. A bit further on, an explosion will sound, sending your cattle off into a running frenzy!
You need to - very quickly - get to the front of the herd and shoot your gun into the air to scare them away from the path of the oncoming train! Unfortunately, the train in question is also in the middle of being hijacked!
A bunch of train robbers don’t appreciate you being a witness to their deed and they try to kill you, so deal with them in the usual John Marston way (by putting a bullet or two in all of them). Head to the front of the train when the first wave are downed to finish off the last group.
Now you can speak to the train driver - if you like - or you can join back up with your Uncle in gathering up the herd before setting back off taking them back to the pasture, completing the mission.