My coleague found in the old archives this interesting document. As you can read on the page 3, the goverment gave the task for Ministery of Transport to manage the electrification of the track Tanvald - Polubny (the names from 40-ties) with the deadline in the year 1960.

Finally, the electrification did not take the place and it was decided to put the diesel locomotives into the service - probably due to expenses. The development of the new engines would be too complicated, demanding too much time and money, so the new locomotives were ordered at austrian company SGP. So the unique diesel cog locomotives were created (T 426.0 series).