Urban rail transportation comprises two main systems: subway and light rail.

Some people mistakenly define subway as underground rail transportation and light rail as above-ground rail, while others associate light rail with smaller, faster passenger capacity rail systems that run on their own tracks but may not be separated from other vehicles.

These distinctions are not scientifically accurate.

Light rail typically refers to smaller, swift rail systems designed to carry passengers. Light rail has dedicated tracks but doesn't necessarily operate separately from other vehicles.

On the other hand, heavy rail is commonly referred to as the subway. These systems are designed to transport a significant number of passengers at high speeds while operating independently from other transportation modes.

Internationally recognized definitions from the UIC (International Union of Railways) help clarify the differences: a light rail train is a conventional steel-wheeled train capable of entering standard mixed-use zones, while a subway is a conventional steel-wheeled train without this capability.

It's essential to note that both subway and light rail systems can be constructed above ground, below ground, or on viaducts.

Although subways often use heavier rails, this choice is primarily for track stability, maintenance reduction, increased cross-section, and reduced stray current. The distinction between the two systems is better determined by their capacity to handle peak hourly traffic.

Subways can accommodate a peak hourly flow of 3-6 million passengers in one direction, while light rail can handle 1-3 million passengers in the same time frame.

Differences in design between subways and light rail systems become evident in several ways. Subways typically have axle weights exceeding 13 tons, whereas light rail systems keep their axle weights below 13 tons.

Subway curves usually have a minimum radius of 300 meters, while light rail curves range from 100 to 200 meters. Furthermore, subways operate with more trains per grouping and larger crew sizes than light rail systems.

Another factor in distinguishing between subway and light rail is the selection of train specifications. According to international standards, urban rail transit trains are categorized as A, B, or C, corresponding to widths of 3 meters, 2.8 meters, and 2.6 meters.

Systems employing A or B-type trains with a train set range of 5 to 8 are referred to as subways, while those utilizing C type trains with 2 to 4 train sets are considered light rail.

The choice of train model and the number of sets determine axle weight and platform requirements.

The key differences between subways and light rail are their passenger capacity, axle weights, curve radii, train types, and the number of train sets. Understanding these distinctions is crucial for efficient urban rail transportation planning and operation.