Subject: Re: TTC Tragedy: Blame Everyone Else!

From: [email-masked] (Mark Brader)
Date: Sat, 13 Jan 1996 09:54:06 GMT

> > He ran three red lights, but only two stop signals.  One set of
> > lights was a repeater because of the sharp curve of the tunnel.
> 
> He ran 3 red lights...2 signals...both of which had malfunctioning
> trip-arms, which should have stopped the train if they were properly
> maintained.

The second one would not have done so in any case.  The reason is that
the signal system only detects which blocks have trains in them.  If
two consecutive blocks are occupied, the logic assumes that they are
occupied by the same train, which is at this moment crossing from
one to the other.  Accordingly the trip arm between the two blocks
is kept down, so that it won't trip the train itself.

So long as no red signals are passed, there is always an unoccupied
block between two trains; this block provides enough distance for
a train moving at full speed to come to a stop.  It's the red at
the entrance to this block that is critical; the next red is probably
too close to the train ahead to provide sufficient stopping distance
anyway.

Here is a train moving along:

	#1         #2         #3         #4         #5         #6
1	|----------|----------|----------|--######--|----------|
	G          Y          R          R          Y          R

2	|----------|----------|----------|-------###|###-------|
	G          Y          R          R          R          R

3	|----------|----------|----------|----------|--######--|
	G          G          Y          R          R          R

See how the block to the right of the leftmost red is always an
unoccupied block.

In diagram 2, signal #5 is red but *cannot* have its trip arm raised.
If it did, it would trip the train passing over it.  All the other
reds in each diagram can and do have their trip arms raised.

Okay, now note that I've made signal #6 red, so this train is now
stopping and waiting.  Along comes another train, which has to stop
at signal #4:

4	|----------|----------|---######-|----------|--######--|
	Y          R          R          R          R          R

Only, for some reason, it doesn't, and proceeds into the next block:

5	|----------|----------|----------|-######---|--######--|
	G          Y          R          R          R          R

The system *cannot* distinguish diagrams 2 and 5, so the trip arm
at signal #5 goes down in diagram 5.  Whether it malfunctions or not,
it does not protect the forward train in this situation.  The red
light is still there to be seen by the driver, though.  And, of
course, protection by tripping at this point shouldn't be needed,
because a runaway train passing #4 should have been tripped there.
-- 
Mark Brader, [email-masked] | "In the affairs of this world men are saved, 
SoftQuad Inc., Toronto  |  not by faith, but by the want of it." -- Franklin

My text in this article is in the public domain.