Transportation in the U.S. Railroads are paths of parallel metal rails that allow a wheeled vehicle to move more easily by reducing friction. Iron mils were first used in the early 1800s to guide horse-drawn wagons. After the invention of the steam locomotive (机车) in 1804, steam engines replaced horses as the primary means of power. Modem locomotives commonly use electric motors or diesel (柴油机) engines and pull long trains of passenger or freight (货物) ears. The benefits of rail transportation in both speed and carrying capacity made it superior to other methods of transportation in the 1800s. In the United States and Canada, with their vast expanses of territory, the railroad became a major means of cross-country transportation. In many countries, rail continues to be the main mode of passenger travel. In Europe and Japan, major cities axe connected by high-speed passenger trains, such as the French TGV (Train Grande Vitesse) and the Japanese Shinkansen (新干线) trains, popularly known as bullet trains (高速火车). The Shinkansen train can reach speeds of 300 km/h (186 mph). Railroads have had a profound impact on civilization. Most importantly, the efficiency and speed of mil travel allow a nation's population, industry, and agriculture to be established throughout that nation's territory, even in previously remote areas. Rail transportation has also played a significant role in urban transportation systems. Motor vehicles make up a broad class of self-propelled (机动式的) land-transportation devices that generally use internal-combustion engines (内燃机) and gasoline or diesel fuel for power. Most motor vehicles are designed to travel on roads, but specialized vehicles with more solid construction are used for off-road travel over irregular ground. Motor vehicles range from motorcycles and automobiles for personal use to trucks, which can transport large amounts of cargo, and buses, which can carry many passengers. The first self-propelled vehicle was powered by steam. French engineer Nicholas-Joseph Cugnot built a steam-propelled vehicle in 1769. Between 1860 and 1890, innovations in steam-engine technology led to more steam-powered vehicles, primarily in France. At the beginning of the 20th century, gasoline powered automobiles began to emerge as the dominant form. of motor vehicle. Automobiles and other motor vehicles grew especially popular in the United States. From a relatively modest number in 1900, the number of motor vehicles in the United States reached 458,500 by 1910 and 6.2 million by 1918. At the end of the 20th century, over 130 million automobiles, 69 million light trucks, and 1.2 million heavy trucks were in use in the United States, being driven an estimated 4.5 trillion km (2.8 trillion mi) per year. The heavy truck, which first came into mass production just prior to World War Ⅰ (1914-1918), has become a major means of moving freight. It is a major competitor with railroads for freight business. The bus was developed in response to the apparent popularity of tiding in motor vehicles. In many nations buses are a major form. of urban and intercity passenger transportation. The motorcycle is one of the most abundant forms of individual urban transportation in the world today. The growth of motorcycle or motorbike use has been especially large in Asian cities, where motorcycles provide a means of bypassing crowded automobile traffic. One of the few motor vehicles that do not use wheels is the snowmobile, which uses a drive track (made of a belt of robber) and a pair of skis to move through snow. Although used for basic transportation in areas with heavy winter snows, the snowmobile is mostly used as a recreational vehicle. Over 200,000 snowmobiles are sold annually in the United States, Canada, and Europe. Air travel has revolutionized global transportation by dramatically reducing the time needed to travel great distances. Journeys across nations or oceans that might have taken