Freight train engines rely almost exclusively on diesel. The first over-the-road diesel freight engines entered service in the 1930s and the number of diesel-powered trains in the U.S. surpassed 1,000 in 1940 - most for passenger service.
According to the latest available data from the U.S. Bureau of Transportation Statistics (BTS), at the end of 2018 just over 26,000 freight locomotives were in operation in the U.S., and 431 passenger rail locomotives. With the exception of a few passenger rail lines that are electrified (Amtrak’s Northeast corridor and Harrisburg, PA line), the remainder of passenger rail and all of freight rail in the U.S. is diesel-powered.
While the average car engine today has about 200 hp, locomotive engines typically start at ten times that amount. Train operators rely on diesel power across the full range of rail power applications:
- The smallest locomotive engines (up to 2,000 horsepower) are used in switch operations in freight yards to assemble and disassemble trains or are used in short hauls of small trains.
- Passenger locomotives (typically 3,000 horsepower) are often accompanied by an auxiliary engine for "hotel" power to passenger train cars.
- The most powerful locomotive engines (up to 4,000 horsepower) are primarily used for long distance freight train operations by America’s seven Class I railroads.
Diesel engines have substantial economic advantages over other power sources for locomotives. In addition, diesel locomotives accelerate quickly and run at high speeds with minimal track damage. They function with similar efficiencies as electric locomotives, but do not require the capital investments in substations and electric distribution networks.
The diesel industry and rail manufacturers continue to invest resources and make strides toward producing the cleanest train technology possible. Diesel engine technology in railroad locomotives has advanced dramatically in recent years. Fuel-efficiency has increased 61 percent since 1980.
In 2004, the U.S. Environmental Protection Agency (EPA) released its Final Nonroad Diesel Rule that will require train engines meet strict air quality standards. As part of this standard, trains will achieve low emissions levels that will reduce sulfur emissions by 99 percent. These fuel improvements will create immediate and significant environmental and public health benefits.
At the same time, clean rail standards will also require the use of advanced emission-control technologies similar to those already upcoming for heavy-duty diesel trucks and buses. The availability of clean non-road diesel fuel means that advanced clean diesel technology will reduce NOx and PM emissions by 90 percent in rail technologies.
Today, the transformation to near-zero emissions in locomotive engines for every application is complete, with new engines now achieving the US EPA Tier 4 Emissions regulations for both particulate matter and oxides of nitrogen, utilizing ultra-low sulfur diesel fuel.