The Northern Line (Thai: ทางรถไฟสายเหนือ) is a railway line in Thailand, running between the capital Bangkok (at Krung Thep Aphiwat Central Terminal) and the northern city of Chiang Mai (Chiang Mai railway station). It is the second longest railway line in Thailand at 751 kilometres (467 mi) long, has 130 operational stations and halts, and is operated by the State Railway of Thailand. The line first opened in 1896. Major cities served by the line include Bangkok, Ayutthaya, Nakhon Sawan, Phitsanulok, Lampang, and Chiang Mai.

The line's operations were severely affected during World War II.

Since January 2023, long-distance (Special Express, Express and Rapid) trains terminated at Krung Thep Aphiwat Central Terminal in Bangkok replacing Hua Lamphong station.

As part of the double- tracking project between Lopburi and Pak Nam Pho, the Lopburi Bypass line between Ban Klap and Khok Kathiam was opened on 5 December 2025, with one intermediate station: Lopburi 2 (Tha Wung)

A new 323 kilometer-long branch line, from Den Chai to the Lao border at Chiang Khong, is under construction and is expected to open in 2028.

The Northern Line is entirely single track, except at stations. Track gauge is 1,000 mm (3 ft 3+3⁄8 in) meter gauge. As the train frequency increases, it is becoming increasingly challenging to operate trains running both direction on the single-line track. Double-tracking has commenced between Lop Buri and Pak Nam Pho, and is planned for the rest of the line. ^{[citation needed]}

The Northern Line is not electrified. Regular services run on diesel power. The current maximum operating speed on the line is 100 km/h (62 mph).