Inlays are runes that are used both for decoration and for summoning bosses when placed into the right pattern (see Structures). There are a few different types of Inlays.
After the most recent update, inlays spawned in some worlds.
Redstone inlays are the most basic of inlay. They are used to summon bosses, and can also be used as rails. They will speed up a minecart as it passes over them. Be wary when cornering too fast, as the cart can leave the tracks entirely.
Iron inlays are an intermediate inlay. They are used to summon bosses, and can also be used as rails. When a minecart touches an iron inlay, it will reverse direction. Some of the momentum is preserved, but not all.
Gold inlays are the most advanced inlay. They are used to summon the most powerful of bosses. They can also be used as rails. When a minecart touches a gold inlay, it will attempt to warp to the next gold inlay in the direction it is currently traveling within 8 blocks. This can transport the minecart (and any passengers) through solid obstacles, as it is in fact teleportation. There is a short delay before the minecart can teleport on a gold rail again after it warps.