Iron Horse is a Game Type in Battery Mode. Iron Horse is based around an attacking team and a defending team.
In Iron Horse, one team takes command of an armored train. The team must move the train from one end of the map to the next, clearing obstacles set by the defending team. If the team commanding the train makes it to the end of the map, they win. If the team defending can stop the train or destroy it, they win.
As always, secondary objectives are hugely important, and can range from additional cars added to the train to additional routes unlocked.