I sax a service unit in Shelby last week creeping down the tracks with the logo “Sperry Rail Service” on the side. Here is a picture of the unit from there website. http://www.trainweb.org/elso/SRS134A.HTM
Reading from there homepage I believe they are a private contractor for railroads but what do they do? here is the link to their home page.
Ultrasonic rail crack and flaw dectection.
As they go down the track sensors on the rail look for defects. If the machine picks up something of interest, they stop back up and jelly up the rail like your doctor would to you doing an ultra sound. Thier is a handheld scanner that allows them to do a more detaild exam of the are of interest.
Those yellow cars are getting more and more rare as they go to highway/hirail based machines for easier deployment.
There was one of those in these parts a couple of months ago.
Neat to see.
Csx has had the hi rail sperry truck down here a few times over the past year.
stay safe
Joe