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[email protected] · February 2nd 04, 11:36 PM posted to uk.transport.london

In article ,
(Clive D. W. Feather) wrote:

Hot (sorry) off the press ....

I caught a High Barnet train (train 26, as it turns out) at Leicester
Square this afternoon, boarding the fourth car back. We progressed
normally as far as East Finchley, arriving between 14:30 and 14:35.
Some people later said they'd smelled something, but I can't say I did
(on the other hand, I have a cold).

As we sat there, a member of staff walked in the (platform-facing) door
at the front end, walked along the car, and out the other. About that
point I noticed an odd smell, sort of burning. Others also started
wrinking their noses and commenting. Half a dozen or so decided to get
off, when the doors shut (separating at least one party). Then the
doors opened again, we sat for a bit longer, and then the platform
staff started shouting "all change", so we all got off.

There was smoke oozing out from between the train and the platform
where I was standing, and more coming out the other side of the train.
After three or four minutes the train departed under green signals. On
the negative rail, level with the location of the rear shoegear on that
car, was a nasty-looking burn mark on the rail. Rather odder was that
there is another such mark about 10m south, which is under the *fifth*
car (which doesn't have shoegear).

I forget the proper term for this (I was given a good technical
explanation when I raised this point with somebody some months ago).

There are places on the negative rail where there are two patches, which
correspond to the spacing of the two negative shoes on a car - this would
probably be around 30 feet. When examined close up, the patches show that
they are areas where extreme arcing has been taking place. The patches are
probably around 2 feet long. This generally only seems to occur on the
negative rail and only seems to have occurred since the introduction of
the 95 stock. They do not actually have to line up with the normal
stopping point of a car.

The patches give the impression that the negative rail might be slightly
lower at that point, causing the shoe to break contact with the negative
rail and hence arc. Where there are two patches, this acts like a
miniature rail gap and the car actually comes off current for a second,
enough to give quite a large arc. The patches are generally on the section
of the line where the train would be motoring - i.e. in a platform (when
pulling away) or past the station. Examples are Just north of Golders
Green NB, just south of Brent Cross SB, south of Hendon SB (1 patch with a
rail gap at the same distance as a second patch would be, Burnt Oak NB
platform. Because the car is motoring at that point, it is drawing several
hundred amps. Often these patches are sufficiently long enough to cause
the traction equipment to reset - as happens departing Brent Cross SB,
usually twice as there one pairs of patches and one patch and a rail gap.

If the current rails are wet, there is often very little arcing or no loss
of motors when passing over the patches. This is because the thin film of
water on the rail acts as a conductor between the rail and the shoe.

The link below shows the first pair of patches departing BrentX SB.
Although they look close together, they are two negative shoes apart. The
single patch and rail gap are in the distance and can't be easily seen.

http://www.romilepa.pwp.blueyonder.c...entxsbarc1.jpg

Roger

The following train (21, IIRC) was a Mill Hill East. Everyone got on
and we progressed normally to Finchley Central, where I alighted. The
train continued on normally and, in the north end siding, was what
turned out to be the original train with someone looking under it from
the side.

We then had a High Barnet train come and go, followed by one showing
"Finchley Central" which came into platform 1. Then there was a second
High Barnet, after which train 26 came out of the siding (this would be
about 14:50) and proceeded southwards towards East Finchley - no signs
of smoke.

--
Clive D.W. Feather, writing for himself | Home:
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