Pre-stressed concrete sleepers are a responsible component of railways infrastructure, which replacement after years of service is costly and labour consuming. Intense dynamic loading and changing weather conditions cause the appearance of cracks leading to sleepers’ degradation and weakening. The necessity of monitoring of sleepers’ condition for the development of cracks is an actual challenge. Currently used visual inspection of the sleepers’ surface doesn’t provide information about the cracks expansion in depth and its progress in time. Ultrasonic testing has been proposed for quantification of transversal and longitudinal cracks using time and energy parameters of ultrasonic signals and evaluating fast and slow components of ultrasound propagation. The method was approved in the field study on the Latvian railway by comparative testing of healthy sleepers and sleepers with cracks. The cracks were gradually divided into groups according to their sizes that was evaluated by visual inspection. It was shown that the gradual enlargement of cracks manifests both in damping of propagated ultrasonic signals and changing the ratio of slow and fast wave components towards the prevalence of slow ones. Combined use of both manifestations provides quantitative indicators of the cracking process.