Minecart Upgrade Kit

Using it flips a permanent switch on your character. Every minecart you own from that point on gains the boost, and a toggle beside the minecart slot turns it back off when you want the plain handling. Using a second kit later just hands you a spare Mechanical Cart.

A boosted cart is a weapon. It fires at anything in a wide arc ahead of you with no input from you, it hurts far more when you run something down, and it feeds you defense that climbs with your speed. A long rail line through a Hardmode world stops being a travel convenience and becomes a way to clear the world.

The three pieces come out of mechanical boss treasure bags, so this is Expert and Master only, and you need all three mechs down. Classic worlds cannot reach it at all. If you play Expert and lay any track, it is one of the better reasons to open bags rather than sell them.