Transportation Technology Center, Inc. (TTCI), in Pueblo, Colorado, conducted a study to determine the effect of tie stiffness on the durability of tie plates mounted on plastic and wood ties on the High Tonnage Loop (HTL) at the Facility for Accelerated Service Testing (FAST). Recently, several tie plates mounted to plastic ties have failed under normal operating conditions at FAST, while no tie plate failures were observed in wood tie sections in the same conditions. TTCI completed a metallurgical analysis, a field test, and a laboratory test to examine possible causes of the tie plate failures. Three tie test zones were studied, two plastic tie zones and one wood tie zone. Two zones experienced over 900 million gross tons (MGT) of heavy axle load (HAL) traffic, while one zone had over 420 MGT of traffic. The work discussed here provides evidence that the lower stiffness of plastic/composite ties adversely affected the service life of the tie plates under loading conditions at FAST. Review of laboratory testing and testing at FAST shows that there is a direct relationship between tie stiffness and stress recorded in the tie plate. Test results suggest that plastic ties allow the tie plates to flex more than those mounted on timber ties, which have a higher stiffness, and therefore subject the plate to a higher stress environment. This high-stress, high-cycle load environment caused early fatigue failure in plates supported by plastic ties. A direct comparison of tie plate performance was made between wood ties and plastic ties in a 6-degree curve on the HTL using three separate 100-tie test zones. Approximately 16 percent of the tie plates mounted on one type of plastic tie on the high rail of the curve cracked on the field side of the rail under 39,000-pound (lb) wheel loads during 900 MGT of traffic. In a nearby plastic tie zone, with over 420 MGT, 3 percent of the high rail tie plates experienced identical failures. None of the tie plates on wood ties cracked during 900 MGT of traffic. Because plastic ties are an alternative for wood ties, TTCI recommends continued investigation into broken plates in revenue service.