Comfort parameters in the saloon of passenger train are an important criteria for the clients of passenger train operator – for the passenger. In some cases, this criteria can even be a reason to change transports means in favor or disfavor to railway in the winter time. It is an important goal to maintain stable and comfortable environment in the passenger saloon, irrespective of changes in train load percentage, ambient air temperature and operation mode, to attract as much as possible passengers to the railway transport. This problem is at first important for operators, but the solution of this problem must be found at the rolling stock manufacturers level. This problem is even more complicated for diesel multiple unit (DMU) trains, which are limited in the available energy for heating, and in the same time improving the fuel efficiency, as any increase in heating power will lead to increase in primary energy source – diesel fuel. Solution to this problem is reviewed in this article, in particular as solved on the DR1AC series DMU trains modernised for the Latvian railways, where energy effective, combined air and electrical heating system is used, to maintain full compliance to requirements of passenger saloon environment, as laid out in standard EN 14750-1.