Detector Rail

A detector rail is a type of rail that produces a redstone signal when a minecart is on it.

Breaking
A detector rail can be broken fairly easily by hand, dropping itself as an item. It can be broken faster by using a pickaxe.

A detector rail also drops as an item if:
 * the block beneath it is removed
 * water or lava flows over it
 * a piston moves it into a space with no floor below it.

Usage
A detector rail can be used as a rail that can detect when minecarts are on it or how full container minecarts on it are.

To place a detector rail, use the control on the face of a block adjacent to the destination space.

A detector rail can be attached to:
 * the top of any full solid opaque block (stone, dirt, blocks of gold, etc.), including full-block mechanism components (command blocks, dispensers, droppers, note blocks, and redstone lamps)
 * the top of a hopper, upside-down slab, or upside-down stairs.

A detector rail cannot be attached to the side or bottom of any block, but attempting to make such an attachment may cause the detector rail to attach to the top of a block under the destination space. For example, if a fence is on the ground, attempting to attach a detector rail to the side of the fence causes the detector rail to be attached to the top of the ground next to the fence instead.

If updated while on an opened trapdoor, a detector rail (or other rails) breaks drops as an item. Placing a minecart on a detector rail on top of a closed and unpowered trapdoor opens the trapdoor and updates it, causing the detector rail to break. If the trapdoor is powered while the minecart is placed, the trapdoor does not open and the rail does not break. The minecart on the detector rail powers the trapdoor and keeps it closed even if the external power source is removed, allowing the detector rail to stay on.

When placed, a detector rail configures itself to line up with any adjacent rails (including activator rails, powered rails, and other detector rails), as well as adjacent rails one block up. If there are two adjacent rails on non-opposite sides, or three or more adjacent rails, a detector rail lines up in the east-west direction. If there are no adjacent rails, a detector rail lines up in the north-south direction (but if a rail is later placed to the east or west, the detector rail re-orients itself in the east-west direction even if it is already connected to another rail to the north or south). If a rail it would line up with is one block up, a detector rail slants upward toward it (with multiple options to slant upward to, a detector rail "prefers", in order: west, east, south, and north). Other configurations can be created by placing and removing various rails.

Mobs avoid walking across a detector rail (or other rails), but can be pushed onto them.

It cannot be placed suspended in midair, even with commands.

Rail
Detector rails (and other rails) act as "roads" for minecarts. A minecart that enters a detector rail's space from either end of the detector rail continues to move, losing only a little velocity (which can then be increased again with powered rails). A minecart that enters a detector rail's space from the side turns east or south (depending on the detector rail's orientation), or in the downward direction for a slanted detector rail.

Redstone component


A detector rail activates when any minecart is on it (even if only a portion of the minecart is on it), and deactivates when no minecarts are on it. The duration of the signal is always a multiple of 10 redstone ticks (although lag or unloading/reloading a chunk might affect this). While active, a detector rail:
 * powers adjacent redstone dust and adjacent redstone repeaters facing away from the detector rail, to power level 15
 * powers adjacent redstone comparators facing away from the detector rail to a level corresponding to the fullness of the minecart if it is a Minecart with Chest or a Minecart with Hopper.
 * strongly powers any full solid opaque block beneath the detector rail to power level 15
 * activates any adjacent mechanism components, including above or below, such as pistons, redstone lamps, powered rails, hoppers, etc.

If the minecart on the detector rail is a minecart with chest or minecart with hopper, an adjacent redstone comparator facing away from the detector rail outputs a power level proportional to the container's fullness, possibly power level 0. For any other type of minecart (including a regular minecart with a mob riding it) the comparator's output is zero.

A comparator can read the contents of a minecart with hopper or with chest on a detector rail through a solid opaque block, as it can with other container blocks.

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