Hi Andrew
On (15 Aug 97) Andrew Hamblyn wrote to Scott Pitcher...
 AH> can you elaborate on "exchange" for me please?? (ie, what does it 
an??)
No doubt you'll get a good reply. My description follows. I never did work on
railway track safety/ signalling but having some bad experiences of 
aralleled
technology [Castell locks and such, in HV substations] I paid some attention
to the problems and misconceptions.
The Staff is a curiously fabricated gadget that is usually unique, and its 
ob
is to enable certain operations of track points and signals, while
consequently blocking operations of others.
The staff is "offered" into a mechanism within the signal box and when in
place, it releases some previously locked levers and jams some others. When 
n
place, the staff cannot be carelessly or accidentally removed in such a way 
s
to cause collision or derailment. At the same time as it locks/ releases
mechanical devices, it may also operate electrical contacts which serve
relays, signal lamps, point motors, boom gate motors etc.
On single line working, the passing of trains travelling in opposite
directions is done safely because just as long as the crew of train A are
holding the staff for the section they are in, the points cannot be swung to
allow train B to enter that section. So the staff is a permit to run on that
section of rails. One staff, one train, no collision.
Train A continuing, may enter another single track section, for which a Staff
is required. Exchanging his "used" staff for another he is enabled to 
roceed.
And by that "used" staff being placed into the machine then  train B may
acquire a staff in similar style. Depending on local rules, one or both
trains may stop or crawl through the short section of duplicate track where
the signal box is.
I said I had some bad experiences. At a train depot where I was employed we
had each to service three e.m.u.trains each shift. Each of us had a personal
safety key, and we would put this into a machine so as to release a task key.
As long as any one of the many task keys was missing from the machine, the
trolley wire [1500v] could not be livened.
Before my task was completed I was called to the Supervisors Office [to be
disciplined- yelled at!] and returning to my job I didn't realise that I was
late and that "my" train should have gone and been replaced by another.
Under the train, I opened a motor cover and caught a whiff of hot air, and 
ot
a small shock [static]. Panic...... I got out from under real fast and very
shaken.
What had happened, was that in my absence someone had produced a Master Key 
o
as to release the interlocks and get the train out. My life was on the line,
that train had to roll. I had started work on a live train for which my task
key was not securing my safety as it should.
SO there are no free lunches, no Santa Claus, and no foolproof safety rules.
Cheers.....ALEC.  Alive and kicking!
... If you don't go to other people's funerals, they won't go to yours.
--- PPoint 1.92
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