In-field testing for structural integrity using dynamic non-destructive investigation of ballasted railway track structures was carried out and is presented in this paper. A non-destructive technique, ‘modal analysis’, was utilised in this investigation. The railway track structures were dynamically excited using an impact excitation instrumented hammer. Due to the nature of railway track structures that they extrude for a long distance, the transient excitation was given randomly at several local areas. Acceleration responses in time domain were recorded using an accelerometer mounting on the railhead. The frequency response functions (FRFs) can be obtained via the PULSE FFT Analyser. The analytical formulation and best curve fitting algorithms were developed to identify the dynamic properties of railway tracks’ components. Rather than providing only the absolute solutions, this paper points out more on how to judge the dynamic responses (e.g. FRFs) together with the visual inspection of existing conditions from the field experience. Examples of testing results representing the deficient integrity are additionally highlighted. Based on the results, the impact excitation technique is an efficient method susceptible to the structural integrity of railway track structures.