In this paper, an original method is proposed which allows to estimate the modal parameters of a railway vehicle running on a real railroad line. Based on on-line measured acceleration, the methodology, hypothesising the railway corrugation as a random process, can separate the effects of the forced and the free response of the vehicle. Analysing the axle-box measured acceleration, the Random Decrement Technique provides the free response in terms of autocorrelation function. From the obtained autocorrelation function, with time domain modal parameters identification methods, the natural frequencies and the related damping factors can be estimated by means of Ibrahim and Prony methods without any numerical modelling. In the paper, the comparison between stability analysis using the identified damping factors and the traditional acceleration RMS value, both in stable and unstable conditions, are reported. Experimental results, related to different railway vehicle configurations, are analysed. For the covering abstract see ITRD E117109.