The network constraints of freight railroad traffic are moving the industry to heavier tonnage trains, higher axle loads and increased velocity. The dynamics of heavier axle loads and speed produce high stresses in the track infrastructure. Track integrity is strongly related to the vertical track modulus - the relationship between the rail deflection and the vertical applied load. Both low track modulus and large variations in track modulus increase dynamic loading leading to increased maintenance requirements and reduced structural integrity. Currently, there is no widely accepted method to measure vertical track deflection and modulus from a rail car traveling at speed and no criteria exist to relate variations in modulus to the track integrity. This paper presents a method to measure vertical track deflection from a moving rail car. These deflection measurements can then be used to estimate track modulus. Exception criteria are then proposed that can be used to evaluate track quality and prioritize maintenance. Revenue service test results are presented for approximately 350 miles of the Union Pacific Railroad's heavy-haul axle freight line on the South Morrill and Powder River sub-divisions. Finally, the method is compared to other measurements of ride quality including Vehicle Track Interaction (VTI) measurements on the same sections of track.