N Scale Covered Hoppers

A covered hopper is a self-clearing enclosed railroad freight car with fixed roof, sides, and ends with openings for loading through the roof and bottom openings for unloading. Covered hopper cars are designed for carrying dry bulk loads, varying from grain to products such as sand and clay. The cover protects the loads from the weather. Dry cement would be very hard to unload if mixed with water in transit, while grain would be likely to rot if exposed to rain.

Source: Wikipedia