The mission Carl hands over when he expects to die: rig the train to explode and put it down the pit.
Believing he is finished, Carl gives Princess Donut the plan in three sentences: forget about me, continue with the mission, don't risk the cart. You need to get the bomb into The Abyss. Fire Brandy will walk you through setting The Nightmare Express up to explode. After, go to Elle McGib and Imani C — they'll take care of you.
It is the first explicit statement of what the whole trainyard expedition was for: a train, rigged, dropped into the pit where thousands of monsters, a Province Boss and now a war god's vessel have been piled. Donut's reply is DON'T BE AN IDIOT, CARL. THAT'S NOT FUNNY, and by the end of the fight the mission is still pending with Carl alive to run it.
The endgame plan for the floor, stated for the first time as a handover.