It has been known for some time that open empty cars have a much higher air drag than full ones. In many cases this difference will be big enough to compensate the additional energy needed to transport the additional weight of full cars as compared to empty cars.

Vollmer 1989 gives the aerodynamic values for a number freight cars including empty and full cars. Using the data for the freight car of type Eaos 106, one finds that full cars have 65% less air resistance than empty ones.

• It is safe to assume that providing a cover for open freight cars will at least give the same aerodynamic advantage as filling the car since a cover will normally have a smoother surface than the load (e.g. gravel or coal).
• Therefore for trains consisting of open cars only (which is typical for long trains transporting mass goods such as coal etc.) the air resistance on the routes where the trains run empty would be cut by 65%.
• Since air drag accounts for about 50% of the total energy demand of a freight train and assuming that the additional weight of the cover is negligible for energy considerations, one gets an estimated saving potential of up to 30% - 35%.
• It is difficult to assess the relevance of empty open cars for freight operation in a general manner and their contribution to total energy consumption, because it varies strongly between different fleets.

As a conclusion, covering freight cars could cut air drag by one third in specific trains. An assessment of the overall potential in freight service is difficult.