Railway prestressed concrete sleeper or so-called "concrete tie" is an important component of railway track structures. The dynamic interaction between concrete sleepers and ballast bed has rarely been studied because of the main emphasis placed on global track dynamics. Until recently, the health monitoring of track components becomes compulsory in order to develop the optimal maintenance model. Since railway tracks are normally subjected to dynamic loading, it is crucial therefore to evaluate the dynamic properties of their components. Experimental modal analysis is an efficient technique to evaluate the mechanical properties of any structures based on the non-destructive testing measurements. This paper presents results of an experimental modal analysis of prestressed concrete sleepers in voided in-situ conditions. Two types of prestressed concrete sleepers were provided by the Australian manufacturers. The concrete sleepers were tested using an impact hammer excitation technique over the frequency range of interest: 0 to1600 Hz. Frequency response functions (FRFs) were measured using the Bruel&Kjaer PULSE dynamic analyser. The FRFs were processed using the STARModal analysis package to identify natural frequencies and the corresponding mode shapes for the sleepers. The conclusions are presented on the effect of voids and pockets on the dynamic properties of prestressed concrete sleepers. The void configuration presented is the single hovering. Vibration parameters of concrete sleepers are required for the development of a realistic dynamic model of railway track capable of predicting its responses to impact loads due to wheel burns, irregularities of the rail, so on.