So, the players role-played, reasonably for people who know they are more-or-less apprentices, and then accepted a railroad.
That means you owe them fair warning, at least, if you turn "script immunity" off. You can keep it in, if you like; that doesn't mean they get to WIN, it just means a way for them to escape, or be rescued.
I endorse all the alternate-means posts, eg the post on "equalizers". Odysseus and his crew didn't defeat the Cyclops in a fair fight.
Luke Skywalker got rescued by Han/Chewbacca just in time to take a shot at a weak point, and he got an unexpected Inspiration from a *dead* mentor NPC. Did the player complain about being handed a fiat victory against a CR-mismatch foe?
If Frodo and Sam had ridden eagles into Mordor, would they still get full XP for destroying the Ring?
