Carl's home-fabricated train-car barricades — a giant metal wedge and door-blocks to isolate a car from mobs.
To survive the Iron Tangle's monster-filling trains, Carl engineers reusable car fortifications at his workbench. The Gangway Chock is a giant rectangular sheet of iron — a swordsman breastplate thinned to a fifth of an inch and widened — that wedges into a gangway too big for any monster to slip around; so heavy it'd shatter an unbuffed foot, but Carl can pop it in and out of his inventory freely. He pairs it with exterior 'door blocks' (weight-bench wedges pinned with crossbars through the doors' handholds).
Together they let the party clear and hold a single car. He learns the doors themselves are lethal — a Pollyslog's arms shear clean off against a wedge as they close — and plans adjustable, modular versions once he can measure the cars precisely.
A giant flat iron sheet with rounded edges (from a reforged breastplate), plus tall wedge-blocks braced with crossbars against the sliding doors.
Carl's signature train-siege engineering — barricades that turn a rail car into a defensible redoubt.