Joyce
Good point, but the time tables were still only half minute resolution even
though printed paper!.
The working timetables are prepared partly to attempt to run trains to a
specific timing (which allows crews to be rostered and train maintenance to
be planned), but also to inform the operator which trains had specific
priority over a junction. Timings were usually only given for sites with
signal cabins (later Interlocking Machine Rooms) where there was a specific
choice of route. This was usually at a station, but some "country"
junctions (e.g. Woodford Junction on the Central) also had timings. The
rest of the line would be worked by, in LUL terms, automatic signals
actuated purely by the passage of trains (some signal cabins were left with
their frames almost permanently in King Lever and unmanned). LUL practice
was to signal a station platform (and have a cabin if there was a junction)
using basically 2 aspect signals with repeaters as required. The layout was
somewhat different to main line practice with every signal having a
calculated (if nominal in real life) sighting point and overlap. At the
time of the 32 train service it is likely that the sighting points and
overlaps were calculated to the same "average performance" (nominal rush
hour traction voltage - it varies depending on the number of trains in
section and sub station settings and full load) curve which was a "perfect
driver" curve - full acceleration to max speed, maintain max speed and a
full, single brake application to rest in the platform.
The nominal signalled headway of the Central Line was always stated as 90
seconds which in normal LUL experience should allow running of a 2 minute
service taking into account service variations - normal driving rather than
perfect, traction voltage variations, station stops longer than the defined
value - usually 30 seconds allowed, but Bank & Liverpool Street "may" have
been 40 & 45 seconds). As long as the timetable stated that a train was due
it was telling the signalman to clear the signals - simply reduce the
interval between trains in the timetable to 90 seconds several times in an
hour at each point (not necessarily the same trains at every point) and you
have solved the problem. I suspect (without doing the sums) that 6 off 90
second intervals per hour would suffice. 32 tph was probably pushing it a
bit, in reality, if time was to be kept.
I was heavily involved (from the LUL end) in the resignalling of the Central
Line in the 1990s and, as part of deciding the strategy for the interim
signalling I did a survey of the headways on the whole of the Central Line
and found that a nominal 90 second did apply although most of the inner
areas were closer to 80 second and St Pauls and Liverpool Street were more
like 70 seconds with 30 second station stops. There were, however, several
section without station stops, which calculated to well in excess of 100
seconds (the max was 114 OTOMH) whic was probably due to permanent speed
restriction being imposed subsequent to th signalling and the latter not
being modified. We did the interim signalling with a max headway of 100
secs (although we usually achieved much less) through the "Central London"
area. The result amazed the Operating Department - running a full service
before we did the interim would usually result in late running of 30 to 45
mins at the end of the peak (they relied on cancellations to be anywehere
near time!), but our revisions allowed a full service to run pretty much to
time (and had most trains stopped in platforms if there was a problem), We
decided that this was probably due to our design holding trains in the
platform longer and then getting a clean, unchecked run to the next station
which prevented or reduced the cascading delays which could easily occur
with the earlier layout. I have since simulated it (on a non LUL simulator)
which tended to prove this.
The resignalled Central Line computers were set to receive timetable data in
15 second resolution (although the computer, themselves, ran on a much
faster set of clock cycles), but the timetables were still printed in half
minute resolution! The system worked in roughly the same way as the old
system with trains only being held to time at specific positions (e.g.
junctions) with the "automatic" signalling working on track occupancy in
between.
Peter
PS Clive Feather has a pretty good description of the CL resignalling on his
site.
--
Peter & Elizabeth Corser
Leighton Buzzard, UK
"Joyce Whitchurch" wrote in message
...
Peter Corser wrote:
The half minute timing accuracy was down to the programme (sequence)
machines stepping every half minute. The timetables on the machines ran
in half-minute time (from 0300 to 0300 the next day) with 0 at midday -
half minute time was the most you could do within the limits of a
computer integer (8 bit - +/-32767 IIRC).
A good job they didn't have computers back in the 1920s then - the Central
London Railway used to run its off-peak trains at intervals of one and
seven-eighths of a minute. (Or 32 trains an hour.)
--
Joyce Whitchurch, Stalybridge, UK
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