Re: RFI - Bridges again.

["Derick Wuen" <cullend@webone.com.au>]

The Morphetts wrote in message <378733B9.1D307E75@onthenet.com.au>...
>(Request for Information)
>Thank you everyone who replied regarding double tracked timber bridges -
>your assistance is appreciated and the outcome is that I have decided to
>stick with the prototype and retain my double tracked sandstone bridge.
>
>Now - perhaps someone can help with this question:
>In Queensland, it is not at all unusual on branch lines to see a railway
>line drop down from its given relative level to a lower level to cross a
>stream, and then return to the previous level. This is so particularly
>with cane tramways, but is also prevalent on older branch lines.
>In NSW the crossing of the stream was usually done at more or less the
>same height as the rest of the line, but can anyone think of an instance
>in NSW where the Qld system was in place?
>
>The question is asked because I'm about to build a bigish timber bridge
>on the CCTCR and I'm in design phase. The bridge will be on a curve, be
>about 300mm above high water and 450 above low water (the Coramba Creek
>has a pump system to make it flow) and its setting is particularly nice
>being near a waterfall under palm trees and tree ferns. The "drop down"
>feature would be especially picturesque if it can be done correctly.
>
>Thank you!
>Graham
>--
>The Coramba Creek Timber Company's Railway
>Web Page:
>http://www.onthenet.com.au/~grahamm/cctcr
>ICQ #: 8322797


"All other things being equal" (which they usually are not) railways follow
the "grain" of the country. Water drains to the lowest part of the country
and then follows the least inclined passage to the next lowest portion, and
so on. Thus the lowest graded routes (most effective) follow watercourses.

If the watercourses trend in the general direction the route needs to
follow, then the route will stick with watercourses until it needs to cross
a watershed into another drainage system. This is described as running with
the grain of the country. The crossing of the watershed is usually done in
rougher country than the rest of the route. Two examples: the NSW short
north sticks with Mullet Ck, and its headwaters up to Woy-Woy tunnel where
it crosses the watershed into the next drainage; the Monaro railway from
Queanbeyan to Goulburn follows the left bank of the Molonglo up to Burbong
and crosses the watershed to the Lake George drainage at Brooks Bank tunnel
after wandering about in rough country.

However, if the route has to run across the grain of the country, it has to
negotiate a series of watershed heights and watercourse valleys. An
expensively engineered route will cut or tunnel through the heights and use
embankments or bridges to cover the dips in an attempt to maintain level
track; a cheaper one will have to negotiate the peaks and troughs as grades,
not in engineering works. The NSW North Coast line and the Qld Sunshine
route are examples of across the grain routes tending towards the "cheap"
rather than expensively -engineered end of the spectrum. Inevitably there is
a bridge at the bottom of each dip, and the only questions are how deep is
the dip and how big the bridge, and its often a trade-off between the two.

An outstanding exception to the rule above was the NAR NG route Maree -
Alice where many watercourse crossings went down one side of the bank,
across to bottom of the watercourse, and up the other bank. This saved
expensive bridges which would be useful only on the "once in a blue moon"
occasions when the watercourses actually flowed. Cheaper to replace the
washaway than build a bridge. Of course the one and only time I Ghaned to
Alice, it rained!

So I don't think its a question of Qld by comparison with NSW, but a
question of the standard of construction adopted when across-grain routes
were engineered. I think you can safely have either a little bridge at the
bottom of a big dip, or a big bridge and little or no dip, depending on the
"character" you wish to project.