I found this information on Wikipedia:
http://en.wikipedia.org/wiki/Walt_Disney_World_Monorail_System
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Train safety
Safe train spacing is maintained via a moving blocklight system, referred to as MAPO, installed in the cab of each train.[4] MAPO appears in the top center of the pilot's console and looks similar to a horizontal stop light. There are three lightsgreen, amber, and redand a push-button labeled "Override". The term "MAPO" itself comes directly from Walt Disney, who formed a new company to deal with
Disneyland's transportation system directly from the profits made by Mary Poppins.[9]
Each monorail beam is divided into blocks based upon pylon numbering. The currently illuminated MAPO color indicates how far ahead the leading train is currently located. A green MAPO shows that the leading train is 3 or more blocks ahead, amber means 2 blocks ahead, and red indicates that the next train is in the very next block. A block is roughly between 500 and 1000 feet (about 150 and 300 m) long, although this varies. The start of each block is called a "hold point", as pilots may need to hold their trains at that location until the train ahead moves away. Guests riding in the front cab of a monorail can identify hold points by the yellow reflective tape around a pylon's number and by two yellow reflectors attached to the top outside edges of the monorail beam at that pylon.
For safety, trains must be kept at least two blocks apart during normal operation. A red MAPO indicates that train spacing has become unsafe. When a red MAPO occurs, the train's on-board computer locks out the pilot's propulsion control and applies emergency brakes. The pilot cannot resume control of the train until either the MAPO clears or the pilot presses and holds the MAPO override button.[4]
It is the pilot's responsibility to avoid a red MAPO during normal operation. When the MAPO switches from green to amber, this indicates that the monorail is approaching the train ahead. The pilot must stop the train before crossing into the next block of beam way and hence before the MAPO switches to red. Should a pilot cross the hold point and receive a red MAPO, this counts as a safety demerit against the pilot. If the pilot ever has three demerits on their record, then they will be transferred out of the monorails department and into a different role at Walt Disney World.[10]
Safety tests are performed daily to ensure that the MAPO system is working properly on each train. At the direction of the monorail station conducting the test, each train will intentionally overrun a hold point to verify that a red MAPO occurs and that the emergency brakes activate. Pilots perform tests in forward and reverse when bringing a train onto the system for the first time that day, and a forward test is again conducted mid-afternoon. The indications are called into Monorail Central with the emergency brake pressures.
A red MAPO will also occur when the pilot approaches a section of un-powered beam, a spur line, or a switch beam thrown in the direction of a spur line. Pilots must engage the MAPO override when moving trains through a switch to the spur line. MAPOs occurring due to safety tests, switching, or beam power loss do not count as demerits against the pilot.
