According to the power source, railway locomotives are mainly classified into three categories.

Steam locomotive
The oldest in history, it is driven by steam engines that convert the thermal energy of fuels (such as coal and oil) into mechanical energy. The structure includes a boiler (for steam production), a turbine (for energy conversion), a running gear (for support and transmission), a coal-water car (for fuel and water storage), etc. However, due to its low thermal efficiency (only about 6%-7%), high energy consumption (water needs to be added every 80-100 kilometers and coal every 200-300 kilometers), and severe pollution, it has been gradually phased out. Steam locomotives were discontinued in China in 1988 and are currently only preserved as historical and cultural heritage.

Diesel locomotive
Powered by a diesel engine and driven by a transmission device to drive the wheels, its thermal efficiency (about 30%-40%) is significantly higher than that of steam locomotives, and it has a long continuous working time and is suitable for long-distance operation. The diesel locomotives in China are mainly of the "Dongfeng" series (such as Dongfeng 4, Dongfeng 11, etc.), and they are one of the main models in current railway transportation.

Electric locomotive
Relying on external power supply (obtaining electrical energy through overhead contact lines or power rails) and driven by electric motors, it has advantages such as environmental friendliness (no exhaust emissions) and high efficiency (high power and fast speed), and is the main direction of future development.

Emu (Modern Extended Type)
It is composed of a bullet train (with powered carriages) and a trailer (without powered carriages), and is divided into power-centralized types (such as the "Shenzhou" diesel bullet train) and power-distributed types (such as the "Xianfeng" electric bullet train). The EMU enhances its acceleration performance by optimizing the power distribution, and its maximum test speed can reach over 250km/h. It is the core equipment of high-speed railways.

Development History
Origin and early period (19th century - early 20th century) : In 1804, the first steam locomotive was manufactured in Trivischick, England. In 1825, the Stephenson "Power" 1 pulled the first passenger train into operation, marking the beginning of the railway era. The first steam locomotive in China was the "Long" on the Tangxu Railway in 1881, but it was once out of service due to a ban from the Qing court.
The Rise of Internal Combustion and Electricity (20th Century) : In 1903, Germany's first catenary powered EMU was put into operation; The first diesel locomotive was introduced in the United States in 1925. China began to produce its own diesel locomotives (" Julong ") and electric locomotives (the first electric locomotive) in 1958. In 1964, the "Dongfanghong Type 1" diesel locomotive and in 1969, the "Shaoshan Type 1" electric locomotive went into mass production, gradually replacing steam locomotives.
High-speed and intelligent development (from the 21st century to the present) : In 2001, the "Shenzhou" and "Xianfeng" bullet trains were launched, with test speeds exceeding 200km/h. In recent years, high-speed electric locomotives such as "Harmony" and "Fuxing" have been put into operation, with a maximum speed of 350km/h. At the same time, intelligence (autonomous driving, condition monitoring) and environmental protection (low energy consumption, low emissions) have become the focus of development.