Western Railroad Discussion > Concrete vs. wood tie usage

I had noticed something on the MetroLink line in eastern Simi Valley, California.
Approximately 300 feet (compass) east of Katherine Road the line crosses over a (pre-fab?) concrete storm drain structure.
One line (the original main?) has wooden ties. The passing siding has concrete ties EXCEPT atop the storm drain structure.
What is the rationale for this?
TIA!
...Lorenzo
 

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Good question and a sharp railroader's eye!
I was in charge of a lot of Metrolink's engineering up until 2006 so unless something changed, here is the story:
First, concrete ties require a lot deeper ballast section than wood ties, both becaure they are taller themselves, but also because they are spaced further apart and thus you need a deeper ballast depth to get the loadings coming down from each tie to overlap at the subgrade or bridge deck.  Without that overlap it is each tie's footprint directly on the subgrade or deck. 
So the ballast retaining side walls of our standard bridges were all designed and built for the depth of ballast need for wood ties, but were (are) too short for the deeper concrete ties plus their deeper ballast.
Second, to replace ties on a bridge it is customary to pick up the whole track panel and lift it out, then install a new panel with new ties.  Wood tie panels are much easier to lift, move, and install.
Third, the track here is not in a significant curve where the added strength of the concrete tie is useful in restraining gauge widening.
When building new track we usually used concrete ties for reasons of added stability, freedom to design our new, deep ballast section, and expected long service life.  If when we purchased main lines in good conditon like here, there was no need to replace a functional wood tie railroad with concrete ties except in heavy curves.
But exceptions abound in this generalization.....

I know this isn't a long bridge, but I've read in TRAINS on the BNSF Transcon that wood ties are mainly used on bridges due to the flexing of bridges. This in turn could cause the ballast to erode the concrete ties much more easily than wood ties. However this rule may not apply to every bridge.
Any thoughts or corrections would be welcome.. Thank you
-Andy

I would think also that wood on concrete has to be a softer pad than concrete on concrete. === === = === JLH

Ballast abrasion also is a factor which I failed to explain above, thanks guys.