A wide variety of cargos are carried on trains, but the low friction inherent to rail transport means that freight trains are especially suited to carrying bulk and heavy loads over longer distances.This use took the form of wagons pulled on wagonways by horses or even humans.Historically, steam locomotives were predominant, but beginning in the 1920s diesel and electric locomotives displaced steam due to their greater reliability, cleaner emissions, and lower costs.They normally operate directly between origin and destination points, such as a coal mine and a power plant, without any changes to the makeup of the freight cars in between.[4] This allows cargo to reach its destination faster, and increases utilization of freight cars, lowering operating costs.