Modernization of railways in Poland should ensure not only safety of passengers, but also limit their negative impact on the environment, especially possibility of train collisions with animals. The aim of the study conducted in the years 2007−2013 on the E20 railway Mińsk Mazowiecki−Siedlce were (1) to record places where animals cross the railway most often, both on the stretches secured and unsecured with sonic deterring devices (UOZ−1), and (2) to specify the number and locations of collisions of trains with mammals. Winter tracking along the railways identified 12 mammal species that crossed the railways. These included four ungulates (moose, red deer, roe deer, wild boar) and eight smaller mammals (red fox, marten, domestic dog, domestic cat, polecat, stoat, brown hare, red squirrel). The average number of tracks of all mammals was higher on the stretches with UOZ−1 than on the stretches over 100 meters away from UOZ−1 (1.52 and 1.01/100 m, respectively, p<0.0001). For ungulates it was higher on the stretches with UOZ−1 (0.66 and 0.38/100 m respectively, p<0.0005) as well. During the study 26 cases of animals kills as result of collisions with trains were recorded: 9 mooses, 2 red deer, 7 roe deer, 15 wild boars, 1 fox, 1 stone marten, 4 dogs. Most of these collisions took place out of the range of UOZ−1.