In article .com,
TheOneKEA writes
(1) The NR rules about transition between 3 and 4 aspects don't
apply. On NR, the two cases should not be mixed and there are
strict rules about how to handle the transition. On LU they are
mixed randomly.
I was already aware of this from a thread RPM had posted in previously.
What I'm more curious about is how the transition logic is handled -
what's the aspect sequence when a driver is confronted with
three-aspect pegs, then four-aspect pegs, then three-aspect pegs again?

On LU, I am not aware of any specific rules.

On NR, it goes something like this. A driver should have a clear idea of
when changing from one area to the other. Therefore there should be a
clear run of several signals of a given type. Acceptable exceptions:
- The signal starting from a terminus or bay is usually 3 aspect even
on 4 aspect routes.
- An extra signal may be inserted into a 3 aspect run just before a
station, and this will require the signal in rear to be 4 aspect.
- When changing from one route to another (diverging or converging
junction, or crossover between slow and fast lines) the driver can
be expected to realize that the system is changing.

The transition from 4 to 3 is simple:

|-O |-O |-O |-O |-O |-O
-------------------------------------------------------
Y R train
YY Y R train
G YY Y R train
G G YY Y R train
G G G G Y R train
G G G G G Y

The only apparent oddity is that 3 aspect signalling comes in *before*
the signal spacing widens out, but at each signal it is clear to the
driver what is meant and the driver always gets sufficient warning.

The transition from 3 to 4 is harder, because of a principle that the
same signal should not use both Y and YY for *start braking here*. So
the "natural" aspect sequences can't be used:

3 7 11 13 15
|-O |-O |-O |-O |-O
-------------------------------------------------------
Y R train
G Y R train
G YY Y R train
G G YY Y R train
G G G YY Y
G G G G YY

because at signal 7 the driver gets Y as a first warning *and* YY as a
first warning, the latter giving 1.5 times the necessary distance to
stop but no marker at the braking distance.

One solution is to make signal 11 approach-released when 13 is red: 7 is
then a 3-aspect signal. This solution is often used where the speed
restriction isn't a problem (e.g. where 11 is the junction signal); it
also means that 11 can be a 3-aspect signal as well (because even if 13
is single yellow a train will have sufficient braking distance after
starting from near-rest):


3 7 11 13 15
|-O |-O |-O |-O |-O
-------------------------------------------------------
Y R train
G Y R train
G Y RY R train
G G RG Y R train
OR G G YY Y R train

(RY means approach-release to yellow).

The alternative approach is to add an extra signal (9 in this case):

3 7 9 11 13 15
|-O |-O |-O |-O |-O |-O
-------------------------------------------------------
Y R train
? R ? train
G YY Y R train
G G YY Y R train
G G G YY Y R train
G G G G YY Y
G G G G G YY

Note that signal 7 can never show single yellow and signal 9 never needs
to show red (it will therefore either carry a triangle on the number
plate, or be numbered 11R). Its aspect may either ignore the train ahead
of it completely, simply tracking that of signal 11, or it may be
controlled to single yellow when there is a train between 9 and 11.

(2) LU has fog repeaters. In multiple-aspect areas, a fog repeater
shows
- yellow if the signal being repeated is red
- white if the signal being repeated is not red.
Does the yellow aspect still have the big black F burned into the lens?

I believe so.

--
Clive D.W. Feather | Home:
Tel: +44 20 8495 6138 (work) | Web: http://www.davros.org
Fax: +44 870 051 9937 | Work:
Please reply to the Reply-To address, which is: