Re: [NSW] Strange rails at Hurstville

[billboltonREMOVE-TO-EMAIL@computer.org (Bill Bolton)]

Graham Fry <fryg@sydpcug.org.au> wrote:

> I am not sure whether this explanation now applies. In the past when the
> city underground was built the additional rails were placed in position
> between the running tracks and electrically bonded to them. Their
> purpose was to reduce the electrical resistance for the return current
> from the train back to the supply substation. It was thought that
> without these extra rail the return current would find alternative
> parallel paths such as pipelines which would lead to rapid corrosion of
> the pipe. 

That only partially correct.... its actually because the rails used on
the city railway were (perhaps still are?) a different type of steel
than normal surface rails.  They were a longer wearing steel, probably
with a higher manganese content, due to the difficulty of replacement
in the tunnels, but involved a trade off of lower electrical
conductivity to get longer wear.  Normal steel rails were laid in
non-running positions in "the four foot" and frequently bonded to the
actual running rails to provide adequate current carrying a capacity.

The same also applied to the curves on the lower North Shore line, but
in recent times tracking relaying in that area has used normal
continuously welded rail and no extra return rails are now provided.
 
Cheers,

Bill