| 1. Semaphore
signaling is an integrated unit with an arm and light
signaling system in which during the daytime the signaling
is indicated by the position of an arm called the semaphore
arm, connecred to a vertical post. During night a coloured
light signal is used to indicate the different signal
status. Semaphore signaling can be two or multiple aspect
type . Colour light signaling is a fixed signaling system
with no moving part and having different coloured lights
to indicate different signaling status ( two or multi-
aspect including 4-aspect types ). Unlike semaphore signaling,
colour light signaling have identical indication during
day and night.
2. In Absolute Block
system, using space interval system, no train is allowed
to leave a specified track section called a block section
unless a ' Line Clear ' is obtained from the block section
in advance, including an adequate distance beyond, known
as the 'overlap', in the form of a permission from the
station masters located at ends of the block section.
In Automatic Block system, the space intervals are secured
automatically by the use of track occupancy detection
systems as axle counters or track circuits.
3. Braking distance is the
distance, a train travels due to inertia, after application
of brakes. Braking distance is a function of train speed,
track condition, brake power, wind velocity etc and
can be considered as around 1 km.
4. Classification
of stations working under 'Absolute Block system' specify
'A' , 'B' and 'C' class stations. In Class-A stations
, line clear may not be given for a train unless the
line is clear upto 400 meters beyond Home Signal or
upto the starter. In Class-B however line clear may
be given for a train before the line has been clear
for the reception of the train within station section.
In Class-C stations permission to approach may be given
for a train not only when the last train has competely
passed atleast 400 meters beyond the home signal, but
is continuing its journey.
5. Fouling Mark indicates
a point or limit on the tracks, with two lines meeting
or crossing each other, beyond which there is an imfringement
of fixed standard dimensions leading risk of side collision
with a train moving in the other line.
6. By fault tolerant
communication is meant that the data communication keeps
reliably working inspite of minor faults in the data
link through automatic software correction or automatic
re-transmission of data telegrams in case problems encountered
in data reception. Alcatel's Axle Counter uses fault
tolerant communictation which allows the system to reliably
function even if the signal cable exchanging data is
not available(disconnected) upto 3 seconds.
7. The terms '2 out
of 2' and '2 out of 3' as applied in Axle Counters and
Solid State Interlocking implies the following ;
In 2 out of 2 system, two processors concurrently operate
and independently arrives at a computed output. Only
when both the outputs tally with each other, the result
is utilised in generating the desired output. In case
of non-agreement the fail-safe logic takes over and
a fault annunciation is inititiated.
In 2 out of 3 system, 2 processors out of the 3 operate
concurrently and vote in a similar manner as in '2 out
of 2' system while the third remains as a standby (often
a hot standby) ready to be brought into operation when
any of the operating processors malfunction. |
Railway
Signaling