Modern train control systems
provide speed commands from the wayside to the lead vehicle by digital
codes electronically delivered via the running rails. "Cab signaling,"
it's called.
The track is divided into blocks. As the train
travels along each block, it receives its speed command for that block
as a repeating bit pattern. Various factors determine the speed limit that
must be enforced on the train when it enters any given block, including
curves in the track and switch settings, station-stops and the presence
of traffic ahead.
Cab signaling must address
several technical challenges...
-
Iron rails constitute a poor medium for communications, imposing
exceptionally slow data rates: often fewer than 20 bits per second.
-
At high speeds, the train can move a considerable distance
during the time required to receive and interpret a new speed code.
-
An inoperative or stuck signal must be detectable by logic
in the cab so that the train can be put into its "safe state" -- stopped.
-
By its own location, the train selects the next block for
reading the speed code, unsynchronized with the signaling pattern in the
rails.
-
All codes must be chosen to assure that the concatenation
of bits in successive repetitions will not be interpretable as a counterfeit
code.
That last requirement mandates the application of what we
shall call "Confusion-Free Codes" and
offers us an opportunity for a puzzle.
Suppose that the following messages are to be communicated
to the train:
-
High Speed
-
Medium Speed
-
Slow Speed
-
Zero Speed
-
Stuck Bits Detection
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