Also see our posting yesterday - Sperry Primer #2.
To answer a couple of questions...we (SRS) have 17 "big cars" or railbound cars in our active fleet. The crew consisting of 2-3 people will live on the car for the duration of the test, going to stores, etc. along the way for food & laundry. The cars are equipped with a shower and a commode. It takes a dedicated person to work on the big cars...lots of responsibilities and work.
The Sperry company was founded in 1928 by Dr Sperry. He developed a technique to shoot an electrical current through the rails and determining by the returning signal if there was a defect there. This was the induction method and is still in use by Sperry an many of its vehicles. Of the other rail testing companies out there, Sperry is the only one to use induction.
This lesson shows one of the earliest Fairmont-built test cars...these were followed by three BRILL cars which didn't work well and were soon retired (and Re-sold to Remington Arms)(some have been returned to service recently by the Edwards Rail Car Co for tourist train service. The Black River & Western has one running as the Delaware Turtle!!!).
Sperry soon standardized on the Winton Powered EMC railcar, buying up many examples from the LV, B&O, SAL, CNW, B&M, RI, NYC, and SOO Line.
The third car pictured is the MACK FCD-railbus that was built for the New Haven. We use this car for subway tests and is semi-permanently assigned to the NYCTA. It can be seen occasionally in transit from the NYCTA to the home office of Sperry in Danbury, CT. Car 403 was rebuilt from the 402 after it derailed and burned in the tunnels of Detroit, MI.
The next lessons will be on the rubber-tired vehicles in our fleet.
SRS 403 in Danbury, CT 
SRS 109 C-B-A one of the first test vehicles
SRS 121 with J.H.DePuy in the late 1930's...coll. of Bob Eilerman
SRS 402 in Trenton, NJ