Track Stiffness Transition Zone Studied with Three-Dimensional Sandwich Track Model

When railroad tracks pass from an earthen subgrade to a bridge, the tracks become uneven. In fact, even if the tracks are level at the time of construction, over time a dip develops. This phenomenon occurs just in front of the bridge and is a cause of greater maintenance and poor ride quality. The track before bridges typically needs to be resurfaced more frequently; this maintenance increases the cost for a railroad owner. The cause for this dip in the track is believed to be the sudden change in stiffness between the subgrade and the bridge. The most commonly used approach to neutralize this sudden stiffness change is called the stiffness transition zone method. The idea is to spread out the stiffness jump from a few centimeters to the length of about 30 m by gradually changing the stiffness of the track. However, the details of the stiffness transition zone—for instance, the length, the pattern of the stiffness change, and the effect of train speed—have yet to be studied. A dynamic track model called the three-dimensional sandwich model was developed to study and compare different stiffness transition scenarios properly. In general, results showed that the transition zone decreased the tie–ballast contact force. An increase in speed will increase the tie–ballast contact force both with and without the transition zone. A longer exponentially increasing stiffness transition zone was proved to be better in terms of decreasing the tie–ballast contact force than was the linearly increased transition zone.