The portable diagnosis system – SPD – evaluates the safety and ride quality aspects of the railway vehicles and the technical condition of the rail-vehicle interface. The objective of this article is to estimate the nonlinear regression model associated with the ride quality or motion behavior, by applying fuzzy clustering algorithms to the geometric data obtained from the technical condition of the railway-vehicle interface and measuring quasi-static lateral acceleration in different vehicles. The performance will be evaluated by comparing the measured acceleration with the acceleration calculated in our model for 15 different vehicles. The obtained results will be then compared with the results of the multiple linear regression model used previously for the same purpose [16].