faster, making them better suited to passenger service, especially underground. Other types Various other types of train propulsion have been tried, some more successful than others. In the mid 1900s, gas turbine locomotives were developed and successfully used, though most were retired due to high fuel costs and poor reliability. In the 21st century, alternative fuels for locomotives are under development, due to increasing costs for diesel and a desire to reduce greenhouse gas emissions from trains. Examples include hydrail (trains powered by hydrogen fuel cells) and the use of compressed or liquefied natural gas. Train cars Train cars, also known as wagons, are unpowered rail vehicles which are typically pulled by locomotives. Many different types exist, specialized to handle various types of cargo. Some common types include boxcars (also known as covered goods wagons) that carry a wide variety of cargo, flatcars (also known as flat wagons) which have flat tops to hold cargo, hopper cars which carry bulk commodities, and tank cars which carry liquids and gases. Examples of more specialized types of train cars include bottle cars which hold molten steel, Schnabel cars which handle very heavy loads, and refrigerator cars which carry perishable goods. Early train cars were small and light, much like early locomotives, but over time they have become larger as locomotives have become more powerful. Passenger trains A passenger train is used to transport people along a railroad line. These trains may consist of unpowered passenger railroad cars (also known as coaches or carriages) hauled by one or more locomotives, or may be self-propelled; self propelled passenger trains are known as multiple units or railcars. Passenger trains travel between stations or depots, where passengers may board and disembark. In most cases, passenger trains operate on a fixed schedule and have priority over freight trains. Passenger trains can be divided into short and long distance services. Long distance trains Long distance passenger trains travel over hundreds or even thousands of miles between cities. The longest passenger train service in the world is Russia's Trans-Siberian Railway between Moscow and Vladivostok, a distance of . In general, long distance trains may take days to complete their journeys, and stop at dozens of stations along their routes. For many rural communities, they are the only form of public transportation available. Short distance trains Short distance or regional passenger trains have travel times measured in hours or even