i’m pretty far from a game designer, but i think as a sort of aperitif - a puzzle-box/gimmick challenge level - it could be quite fascinating as both a design challenge for io (to make it interesting) and a challenge for the player (social stealth in a tight space).
it doesn’t have to be quite as linear as you think. you can get fairly roomy two-decker luxury trains, so you effectively have four vertical planes, including the outside top and bottom of the train (which would be dangerous, natch); and around five horizontal planes (three corridors/rooms inside, and the two outer sides of the train, which are also dangerous).
lots of ways to dispose of bodies; tight confines making social stealth difficult and suit only really challenging; sniper challenge requiring you to time the shot when the train is on a straight so you could shoot down the carriages from one end; lots of potential executions; a time limit based on reaching a destination; stops at stations that reshuffle or add guards; maybe even a gimmick of having to work out when the target gets on - all sorts of potential chicanery.
i dunno, i think current io could make something rather cool with it. 