> Years ago, I was on a tour that stopped at Temora, the SM/Signalman put on
>quite a show in the signal box, explaining things and throwing leavers
>with much 'show'.
> Some then pointed out that one of the crossing bells, down the far end of
>the yard had not stopped, he cursed stormed out, and about 10 minutes later
>came back and reported that some one had put a piece of roofing iron across
>the track, triping the warning bells. It appears that a group of local youths
>had discovered how the signals work, it appeared they were dropping a
>piece of metal of some sort each time they passed through the area.
All this talk of how EASY it is to trigger these circuits is a little
bit counter to what little I know and what little experience I have.
My understanding is that, for low-voltage DC circuits, the short
circuit resistance that needs to exist to divert enough current from
the holding relays to cause them to flip is one of very low impedance
(less than 1 ohm?). This is why track machinery, Sprinter railcars,
even 3801 with sand under the wheels cannot always be relied on to
offer a path of low enough resistance to trigger the circuitry. I
rather think that a 20-cent coin on the track would be hard pressed to
do so. Zinc-plated roofing iron MIGHT do it, however. In Great
Britain, at least, per-way gangs carry "track circuit clips" meant to
short out the rails for safety purposes. They are designed to make a
very firm connection with the shiny parrt of the rail and a force of
over 100 kg per sq cm is said to be necessary at the (necessarily
small area) interface to lower the impedance far enough. Experience
shows this to be true.
Geoff Lambert