Re: Signalling queries

["B." <b.spammed@four12.net>]

"Bradley Torr" <truenorth@one.net.au.SPAMTRAP> wrote in message
01c0f26a$c2b6f800$5c5e65cb@default">news:01c0f26a$c2b6f800$5c5e65cb@default...

> OK, I had the wrong concept in mind. I thought it was as simple as a
> train coming along, and a wheel making contact with a switch of some
> kind attached to a rail, and that switch was connected to the
> signalling system.
> So, when a train enters a track circuit, the wheelsets and axles form
> part of a circuit?

Yep.

> What voltages are we talking about for a typical track circuit?

A 'plain' DC Track Circuit will have a 2v Battery, and somewhere around 0.5v
between the rails at the relay end.  The other end of the scale is a high
voltage impulse track circuit,which has a very brief peak of (off the top of
my head) 50ish v.  This is particularly useful if you have a track circuit
over a piece of track that is rarely used, as the voltage assists in
breaking down the oxide.

> Does the current of a track circuit going from one rail to the
> other via the wheels and axles have any effect on the train or on-board
> equipment?

Not that I know of.  The problem is actually the other way, you can get
problems where traction return current might falsely energsie a track
circuit with a train in the section.

> When no train is occupying a track circuit, is there any
> connection or current between the circuit on one rail and the circuit
> on the other rail?

http://www.trainweb.org/railwaytechnical/sigtxt1.html#TheTrackCircuit

> How long is a typical track circuit anyway? I would guess that it would
> have to be at least as long as the maximum distance between any two
> wheelsets on any piece of rolling stock on Australian railways, so
> while a train is passing over a track circuit, there will always be at
> least one pair of wheels occupying the circuit.....

How long is a piece of string?

Basically they are as long as they need to be, up to a certain limit.  This
limit varies depending on what sort of track circuit is used, how clean your
ballast is etc..

--
B.

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