Two embankments with track performance problems related to embankment instability were investigated. Both sites demonstrated potential for GPR to identify substructure instability resulting in track settlement. The capability to non-destructively evaluate track condition and diagnose the problem cause will ensure that ensuing maintenance addresses the root problem cause, thereby enhancing safety and maintenance efficiency. Two characteristics of the GPR data from these two track performance problem locations were identified as potential track condition indicators. These characteristics were applied to GPR data from a third location with observed performance problems. The application of the condition indicators demonstrates potential for simplifying data interpretation. (C) 2003 Elsevier Science Ltd. All rights reserved.