The Long Causeway

You can't have a second copy on board. Once you have claimed one, you can't launch another, make progress at your opponent's copy or claim it: it is scored only once.

Exhaust a ready Engineer of yours to use it, one that came on board this turn too: it is this card's price, not the Engineer acting (D-139). It stays exhausted until your next stoke phase, so it isn't dispatched or defending meanwhile. Each Engineer pays once; with several, use it once for each. Only you, who launched it, can use it, in your own watch with the crossfire empty; it resolves at once. The progress is yours, outside any battle, so your opponent can't stop it, and no crew adds to it. When your progress reaches its distance, you claim it at once, in your watch (706.7).

You choose X when you launch it, at least 1, and pay it as generic steam (601.3c); it is fixed from then on, on board and in the logbook. With X = 3 it has distance 15 and gives 3 spoils. Crew dispatched here on their own turn make progress, whoever launched it; on the opponent's turn they can only fight (706.3a).