Heavy rail, distinct from light rail, involves the operation of large-weight vehicles on tracks.

The term not only refers to the weight of the tracks themselves but also entails a specific relationship.

Due to the considerable weight of the vehicles traversing the tracks, heavy rail necessitates thicker and sturdier rails, with higher specifications compared to light rail systems.

In urban rail transit, heavy rail systems encompass subways, city railroads, city express rails, and intercity railroads, differing significantly from light rail and monorail systems.

Notably, the technology behind heavy rail trains is entirely distinct from that of light rail trains, as heavy rail systems operate on international standard double tracks with a gauge of 1,435 mm, employing automated signaling systems.

In European countries such as Austria, Switzerland, and Germany, the term "heavy rail" is equivalent to "Vollbahn," contrasting with "Kleinbahn," which denotes narrow-gauge railroads. Although Vollbahn is not commonly used in these countries, Kleinbahn is specifically employed for narrow-gauge rail networks.

Across the United Kingdom, heavy rail primarily refers to the conventional railways constituting the national rail network. This includes commuter, inter-city, high-speed, regional, and freight railways, distinguishing them from subways, light railways, trams, and similar systems.

In North America, the American Public Transportation Organization (APTO) defines heavy-rail systems as electrified railroads capable of accommodating large passenger volumes. This categorization serves to differentiate heavy rail from light rail systems, which have smaller passenger capacities.

Additionally, heavy rail systems in North America typically boast higher train speeds and separate rights-of-way.

In the Philippines, the North-South commuter rail line is described as a "heavy rail subway line" by Junn Magno, General Manager of the Philippine National Railway. While the local definition of heavy rail encompasses subways, the term predominantly refers to electrified train services with eight or ten carriages.

Similarly, in Hong Kong, heavy rail primarily refers to local rail lines operated by the MTR, excluding high-speed rail and intercity passenger services.

Internationally, the International Union of Railways (UIC) defines heavy railways as freight and passenger railroads, encompassing commuter, regional, intercity, and high-speed railroads but excluding high-capacity metros.

These heavy rail systems are typically constructed robustly to accommodate heavy trains, including freight coaches and high-speed trains, and are often separate from road transportation networks.

Heavy rail systems, also known as regional or commuter railroads, are built to specific standards, primarily utilizing steel-wheeled steel rail technology.

While some exceptions exist, such as the Paris Metro's rubber-wheeled rail system for enhanced acceleration and deceleration, and certain urban monorail systems in Japan, heavy rail systems predominantly rely on traditional railroad technology, featuring steel wheels and rails.

Heavy rail systems play a vital role in urban transit, offering robust, high-capacity transportation solutions that are distinct from light rail systems. Through standardized tracks, automated signaling, and efficient operations, heavy rail systems facilitate the efficient movement of passengers and goods in urban, regional, and intercity contexts.

In addition to their primary function of facilitating urban transit, heavy rail systems also play a crucial role in supporting regional and intercity connectivity. These systems form the backbone of transportation networks, enabling seamless travel between major cities and metropolitan areas.

Furthermore, heavy rail infrastructure often serves as a vital link for freight transportation, facilitating the movement of goods and materials across vast distances.

The reliability, efficiency, and capacity of heavy rail systems make them indispensable components of modern transportation ecosystems, contributing to economic growth, accessibility, and sustainability on both local and global scales.