Minecraft wiki entry for Minecart_booster, associated text: A minecart pair will boost when they are side by side.
As long as the carts are side-by-side, they will continue to accelerate, until reaching 8 m/s.
Despite not exceeding this speed, they will continue to gain energy, illustrated by the fact that a longer booster is required to climb a taller hill or mine shaft without eventually slowing down and reversing.
It is ideal to have the booster cart reset itself after use (i.e. return to its original position) so it can be used again right away, but this is not required.
The south-west rule or other track layout techniques can be used to accomplish this reset.
When boosting an unoccupied minecart it will travel about 9 tracks per booster length on level ground (not including reset loop)
When boosting an unoccupied minecart it will travel about 6 tracks up per booster length (not including reset loop)
An occupied minecart will go roughly 12 times an unoccupied minecart on level ground (110 per booster track)
An occupied minecart will go roughly 3 times an unoccupied minecart upwards (16 per booster track)
Stacked boosters (i.e. when more than one cart exists in one space) multiply the boost by the number of stacked carts
Boosters on both sides of a track going the same direction boost by roughly an additional ⅓
Boosters in a series (one after another) boost by roughly an additional 5/9
A cart will slow down if it approaches a neighbor cart head-on (e.g., not off of a slope or curve), and a proper boost will not be accomplished. The sides must come into contact through diagonal or vertical motion. Keep this in mind when designing two-way systems. The following diagrams illustrate this concept: