They're part of the Westinghouse FS2500(I think) track circuit. It's a common
config for a "VF jointless track ciruit". There are no block joints down
there now, save when they might be needed for traction changeovers or for
passing over junctions. Over junctions they usually use a CSEE pulse track.
The triangular boxes are called "data-pick-up-units". They effectively split
a single track circuit(traqnsmitter - rails - receiver) into many
tracks(transmitter - rails - receiver - receiver - receiver, etc.) The'yr
also know as pin-point detectors, which shoulod start to make their purpose
clearer.
When a train approaches, it will hit the main receiver first, thenin sequence
it will short the rails past each of the DPU's(which work by having a signal
induced into them from the rail at the frequency of the track cct.)
This allows you to know the exact position of the first wheels of the train to
within(I think) 300mm. That's pretty accurate, and in some ways more
accurate than the old way, where having one axle on an ac track often won't
cause it to shunt.
I think if you look on the westinghouse site (www.westinghouse.co.uk or
somesuch) you'll find a description of the operation of these.
In the resignalling in the underground, ther's apparently at least on dpu on
every track, and at leaast one intermediate train stop between every pair of
signals. I don't know how true that is, but the old ystem only had
intermediates in the platforms and at some approaches to platforms.
HTH
Shaun
-----== Posted via Deja News, The Leader in Internet Discussion ==-----
http://www.dejanews.com/ Now offering spam-free web-based newsreading