Re: Why Wooden Sleepers are better. Was [NSW] CityRail Delays Friday

[craigd@lios.apana.org.au (C. Dewick)]

The Railway Rasputin II <bob@fastlink.com.au> writes:

>Wooden sleepers don't weigh as much and therefore are less lightly to
>sink and deform the rail. (smooth ride) 
>They don't crack.
>They don't echo flat wheels as much as concrete.
>Most importantly, if the train derails the wooden sleepers break/give
>way so that the wheels don't bounce up and down which can tip the
>carriage. 

They are also smaller dimensionally, which is why a lot of bridges can't
have concrete ties used in place of timber.

The bridge over the Georges River at Como was where that showed up. It has a
ballasted deck and concrete ties. However, the depth of the ballast bed
isn't enough to give good support to the concrete ties, so the track suffers
from a lot of span-bounce-induced track 'pumping' as the ballast bed can't
absorb all the energy that train motion is transferring to the track.

Also the short bridge over the old Princes Highway at Sutherland on the
start of the Cronulla branch. Concrete ties were actually laid over the
bridge but they had to be removed since they were too high.

>The only real advantage concrete has over wood, is that wood has to be
>replaced more than concrete. 

It's got other advantages too, but it's very location-dependent. Concrete
ties *require* that the subroadbed be carefully constructed, especially
where the sub-soil is very clay-based since that's where a large proportion
of drainage problems come from.

So many retro-fits of concrete replacing timber give problems after a few
months because the ballast bed and more importantly, what's under the
ballast, isn't taken into account.

Now you will see most major track upgrades requiring the sub-soil to be
excavated about a metre below where the railhead level will be. There are
layers of 'bombo dust' and sometimes a fabric-like material laid down on the
freshly-excavated strata base, then the track is built (most times in-situ
unless it's a short section being upgraded), then the ballast is built up
over a few passes with ballast trains, tampers, regulators, etc. until the
desired ballast bed depth is reached.

I'd have to speak to a track gang leader to get more info..

Regards,

Craig.
--
            Craig Ian Dewick            |       Stand clear - jaws closing
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