Powered Rail

The Powered Rail is a block added in Version 1.5. It was added to replace bug boosting, but boosters still work as of Beta 1.5. It's operated by Redstone currents. In its unpowered state, it acts as brakes for any minecart which crosses its tile. In its powered state, it will reset the momentum of any cart to pass over it to a reasonably high speed.

Crafting
Powered Rails are made somewhat differently from normal rails; using gold instead of iron, and adding redstone to the bottom.

Boosting Principle
When a minecart passes a powered rail, it will be boosted. If a powered rail is unpowered, it will stop the minecart. Powered rails act like redstone wire, and will pass the power to the next adjacent rail for 9 blocks from the source.

A powered rail boost gives an occupied cart enough momentum to travel 64 tiles on a flat surface from a standstill, or an unoccupied cart 8 tiles. Having more than one powered rail next to each other increases the cart's momentum further, however you won't get much additional boost. It appears to respect the 8m/s travel speed limit that traditional cart boosters don't, so a slower minecart gets a larger speed boost and vice versa, thus it is usually unoptimal to place powered rails next to each other, and instead better to space them out at regular intervals.

The optimal use of powered rails seems to be to put one every 16 blocks for a satisfactory speed along the track once the minecart has some initial speed (that isn't bugged past the 8m/s by traditional boosters), or one every 32 blocks to get a speed that is still acceptable while not consuming not much gold. The player will need to decide on time taken to travel versus amount of resources used when setting a powered rails repeating interval (how far apart a single powered rail should be placed every X tiles).

A powered rail on a slope can act as a launch pad for a minecart. When unpowered it will keep the minecart in place, and when powered, gravity will pull your cart downward, triggering the boost.

A detector rail next to a powered rail will power the rail while going over it, so that way you can have them powered without it taking up any additional space. If you put the detector rail on one side of the powered rails it will mean that the powered rail will boost you if coming from one direction, but stop you if you're coming from the other, which is convenient to prevent runaway carts from going too far, or on a multiplayer server to make sure people use the rail in the right direction. If you want the rail to be a 2-way rail, you can put a detector rail on both sides of the powered rail, or simply place a redstone torch adjacent to the rail. Placing alternate powered and detector rails up a 1/1 slope will not propel a cart of any kind more than 3 blocks upward due to a lack of speed, which is required to clear the booster before it returns to the "off" state. The cart will become stuck unless it is in a "train" of two or more carts, in which case the last cart in the train will become stuck.

Powered Rail Additional Properties
There are currently no curves for powered rails, however, regular rails still prefers to curve towards the powered rail. In cases such as these, the south-west rule applies.

A cart traveling on a powered rail that collides with an object (wall, single block, player, other cart) will reverse direction. It will not reverse direction if it collides with an translucent block, such as stone slabs or glass. If a track including powered rails is bordered by blocks acting as "buffers", the cart will indefinitely continue back and forth along the track. Having carts interact with each other on a short track designed this way can be used to chain multiple carts together as a "train". Once alligned, they will all move together at relatively the same speed.

A cart placed next to a solid object (stone, dirt, sand etc.) on a power rail without power, will start moving when the power is turned on. This makes it possible to easily remote start mine carts, or even to build mine cart junctions.

Bugs
If a Powered rail is only powered by another Powered rail diagonally up or down, you can destroy the rail powering it and it will continue to look and act like it has power until the block is updated. This can be used to have little to no power sources in your track design and up to 2/3 less powered tracks for uphill parts. It acts as a normal rail on the slope but can still stop carts. This means it can be used to obtain the momentum required to be boosted by other powered rail to create a good boosting system, as seen here.

Trivia

 * Powered rails will always show as powered in the inventory even if destroyed and collected while it was unpowered.