For his study, Preonas used the drop in natural gas prices over the past decade as a natural experiment for understanding how market pressures effect the price of coal-fired power generation. By analyzing data on coal deliveries, rail carrier use of the U.S. rail network and hourly energy generation from power plants, Preonas showed that as competition from natural gas forced coal fired plants to reduce electricity prices, railroad companies reduced their coal transportation fees. By absorbing some of the cost difference between coal and natural gas, the railroads propped up the coal market to avoid losing business.