Rapid Rail Sdn Bhd is the operator of the rapid transit (metro) system serving Kuala Lumpur and the Klang Valley area in Malaysia. A subsidiary of Prasarana Malaysia, it is the sole operator of five rapid transit lines which collectively form the Rapid KL rapid transit system. The system currently consists of three light rapid transit (LRT) lines, two mass rapid transit (MRT) lines and a monorail line, with another MRT and LRT line currently under construction.

The LRT (narrow profile) and MRT (wide profile) lines operate on standard gauge (1,435 mm (4 ft 8+1⁄2 in)) rail, while the KL Monorail operates on an ALWEG straddle beam. Train services operate from 6:00 a.m. and typically end before midnight daily, with frequencies varying from approximately three minutes during peak hours to fourteen minutes during non-peak hours.

Malaysia's efforts to revolutionise commuting began in 1984 when the then Minister of Federal Territory Shahrir Abdul Samad released details of a light rapid transit (LRT) system implementation study for Kuala Lumpur. Many were skeptical over its implementation, especially when financing was a major question. The study, undertaken by a Belgian consortium in association with Spie Batignolles of France in cooperation with Master Carriage Malaysia Sdn Bhd, was for an urban or suburban LRT system - powered by overhead electric lines or catenaries. The system - covering 18 stations between Petaling Jaya and Sentul - was targeted for implementation in 1984 and completion in 1988. Unfortunately, the project did not take off until 4 years later, when the Government of Malaysia signed an agreement with Sistem Transit Aliran Ringan Sdn Bhd (STAR). The first LRT line, named STAR LRT after its owner, was constructed and opened completely in 1998. The original system consisted of 36 stations along two routes, Sentul Timur – Ampang and Sentul Timur – Sri Petaling.

Around the same time, the government signed another agreement with Projek Usahasama Transit Ringan Automatik Sdn Bhd (PUTRA) to construct the second LRT system. The line, known as PUTRA LRT, featured a single route from Gombak in the north to Kelana Jaya in the south. The section of the line was built underground and became Malaysia's first underground railway. The line was fully operational in 1999, and complemented the STAR LRT system.

However, both STAR and PUTRA ran into financial difficulties and both companies filed for bankruptcy. In 2002, both companies were bailed out by the government, and the newly formed Prasarana Malaysia took over ownership of both the lines, renaming them STAR Line and PUTRA Line respectively. In 2004, the routes were renamed and rebranded under the Rapid KL brand, with the STAR Line producing the LRT Ampang Line and LRT Sri Petaling Line, and the PUTRA Line forming the LRT Kelana Jaya Line. Operations of the three lines were assumed by Rapid Rail. In 2016, the Sri Petaling Line and Kelana Jaya Line were both extended beyond their southern termini to Subang Jaya and Puchong respectively, with both lines now sharing a common southern terminus at Putra Heights.

In the 1990s, the government of Malaysia commissioned a straddle-beam monorail to serve the city centre of Kuala Lumpur. Various delays and changes to the project occurred, however, the KL Monorail line was fully operation in 2003. The Line was owned and operated by KL Infrastructure Group. The company ran into financial difficulties, leading to a government takeover. In the end, the line was sold to Prasarana Malaysia, and was placed under the operations of Rapid Rail and was integrated into the Rapid KL system in 2007.

In 2010, then Prime Minister Najib Razak announced proposals to introduce a new mass rapid transit (MRT) system to complement the current metro system. The proposal included three new lines in a "Wheel and Spoke" formation around the city. The Mass Rapid Transit Corporation Sdn Bhd (MRT Corp) was founded in 2011 and was appointed as the owner of the project and the MRT lines, unlike the LRT and monorail lines which are owned by Prasarana Malaysia. However, Rapid Rail was awarded the concession to operate the MRT lines, thus allowing the MRT lines to be full integrated with the Rapid KL system. The first line to be constructed, the MRT Kajang Line, which runs from Sungai Buloh to Kajang, entered full revenue service in 2017.

The rolling stock of the Kelana Jaya Line, in use since the opening of the line in 1998, consists of a fleet of 35 Mark II Bombardier Advanced Rapid Transit (ART) trains with related equipment and services supplied by the Bombardier Group. The ART trains consist of two-electric multiple units, which serve as either a driving car or trailer car depending on its direction of travel. The trains use linear motors and draw power from a third rail located at the side of the steel rails. The plating in between the running rails is used for accelerating and decelerating the train. The reaction plate is semi-magnetised, which pulls the train along as well as helps it to slow down.