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 Inlay[]
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 Inlay[]
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 Inlay[]
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.