At 17:20 28/07/99 -0700, you wrote:
>Michael Rigby-Jones wrote:
>>
>> I used to work for ABB Transportation who made these things so I should
>> really know....but it's been a while :o)
>>
>> I *think* the yellow aspect meant something like the next block section but
>> one is occupied so proceed with caution?
>>
>> Also the red aspect is on the bottom, so that there is no hood from another
>> aspect underneath that snow could build up on and obscure the signal.  I
>> think.
>>
>> Cheers
>>
>> Mike Rigby-Jones
>>
>
>Wow - the red on bottom is snow related is interesting.
>
>OK - Rr signals are one of three things (simplifying a bit).
>They can be block signals, CTC signals, or interlocking signals).
>
>block signals have 3 aspects, red means stop then proceed, yellow means
>basicly that the *next* signal is red, so go slow enough to stop before it,
>and green is clear.  They are simply an additional safety appliance, and
don't
>overrule train orders.
>
>CTC signals are operated from a central console, and do replace train orders.
>Instead of written train orders, the dispatcher manually sets blocks to green
>to allow trains through (they automaticly snap back to red when it passes).

The "DRIVER" doesn't know the difference between a CTC section and a BLOCK
section, unless written train orders are in hand. Signals DO override Train
orders, unless that order has been pased by the owner of the section to
"Ignore that ONE" signal i.e. An order must be given for each signal point
within the section.

You should see the mess when you try to go the wrong way down a section.

>
>Interlocking signals are at junctions, and signify the position of the
various
>track switches and whether various tracks are cleared through the junction.
>They used to be controlled by guys in towers using cool mechanical computers
>as a safety check, but nowadays they're usually controlled by CTC.
>
>The computers (drifting back towards PIC topic), acutally logic machines,
>called interlocking plants, used a system of vertical bars connected to
>manual levers, and a system of "dogs" - horizontal sliding bars - to
>"interlock" them.
>It was interesting because it was a 'change' system. Each lever in each
>state would lock or unlock some of the other levers.
>      If A, then B and C cannot change state
>      If B, then A and C cannot change state
>Levers controlled signals and turnouts. Some of the levers were controlled by
>treadle switches activated by trains, the rest by the operator.
>
>Most of the logic was of the 'prevent a bad thing' variety -
>Don't allow a switch to be thrown while a train is detected in that block.
>Don't allow a signal to be cleared while a turnout is thrown against the
track.
>
>Most of the alternate uses of yellow are in conjunction with interlocking -
>various flashing yellow/red combinations indicate safe to proceed at various
>speeds through diverging route.
>



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