Over the past years, a problem associated with sudden loss of braking has been observed in winter conditions for trains equipped with tread brakes and composite brake blocks (CBB). The problem is more significant at temperatures below -15 °C and in combination with fly-off snow. This problem is not occurring for trains equipped with the traditional cast iron brake blocks. Due to the severe nature of this problem and potential risk for incidents and accidents, the Swedish Transport Agency (Transportstyrelsen) is carrying out different activities to find solutions. One such activity is a more detailed understanding of the friction mechanisms between brake block and wheel, with special emphasis on the effect of sub-zero temperatures as well as presence of ice/snow/water between brake block and wheel on the friction behaviour. Furthermore, it is also of interest to find test methods that can be used for evaluation of new developed brake materials under well-controlled laboratory conditions. One observation from field tests is the presence of an ice layer on the friction surface of the brake block. This is believed to be one cause for the reduced friction and hence loss of braking performance. It is also of interest to understand if different brake materials have different probability of forming ice layers due to their water absorption properties. The work has been carried out at the Division of Machine Element, Luleå University of Technology (LTU), and was commissioned by Transportstyrelsen.