Why do they clean the ballast?

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RCH

Been Nothin' Since Frisco
#2
Ballast not only supports the track, it allows for water to drain through it. It's like a filter and it gets clogged over time. Once the ballast is clogged, it holds water and forms mud. As a train passes over the rail, the weight of the train will pump the mud through the ballast and weaken the roadbed. Once the roadbed is weakened, it's not too long before the track is out of alignment. If you look at an aerial photo of main line, you can find the places where mud has been pumped by the passing trains (very common near grade crossings).
 
#3
Some railroads lay asphalt or what's often referred to as "geotextile" on the subroadbed prior to ballasting to prevent mud from forming under the ballast. The recent double-tracking of Abo Canyon on BNSF's Transcon route used asphalt on the subroadbed of the new line, which also provided a paved route for the B&B and signal crews as they did their work along the ROW.
 

BNSFEng

Locomotive Engineer!!!
#4
In addition, they are removing a layer of old ballast that is no longer able to hold the track firmly. After months and months of pounding and moving around under heavy trains, slightly, the sharp edges of the ballast get rounded so they start to slide past each other instead of locking in place with the sharp edges. I just heard about this a couple months ago. That's why they can't reuse the ballast they remove even if they clean the mud off of it.
 



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