Runaway Carriage

A 5/6 trampler for four mana would be a serious rate on any board, and that overstatement is the whole point: the carriage is a battering ram you get to swing exactly once. The sacrifice clause fires on attack or block, so the moment the construct enters combat it is spending itself. That is what reins in the obviously-too-good statline. Leave it home and it will happily park as an oversized artifact body, but the instant you commit it to the crash (where trample pushes damage past whatever chump-blocks in front of it) or throw it under an attacker, the trigger is armed and it dies at end of combat. The card refuses to settle into the role most big bodies play: a wall that controls the ground turn after turn. It can hold the line for one exchange, never two. That constraint points to an honest design tension, an artifact with no color requirement and a body well above curve, built to detonate rather than to dominate. The flavor lines up cleanly with the rules: a driverless coach is momentum without control, useful for exactly one collision before it wrecks. Decks that can turn a one-shot construct into ongoing value (sacrifice payoffs, reanimation, ways to blink or recur it) get more mileage than the blunt body suggests. Absent that support, the carriage is a single swing of borrowed power, priced like an artifact precisely because using it uses it up.