The rail transport system in Denmark consists of 2,633 km (1,636 mi) of railway lines, of which the Copenhagen S-train network, the main line Helsingør-Copenhagen-Padborg (at the German border), and the Lunderskov-Esbjerg line are electrified. Most traffic is passenger trains, although there is considerable transit goods traffic between Sweden and Germany.

Maintenance work on most Danish railway lines is done by Banedanmark, a state-owned company that also allocates tracks for train operators. The majority of passenger trains are operated by DSB, with Arriva and Nordjyske Jernbaner and Midtjyske Jernbaner operating on some lines in Jutland. Goods transport is mainly performed by DB Schenker Rail, although other operators take care of a significant portion of the non-transit traffic.

Denmark is a member of the International Union of Railways (UIC). The UIC Country Code for Denmark is 86.

The Kingdom of Denmark's first railway opened between Copenhagen and Roskilde in 1847. The first railway in the Danish Duchy of Schleswig opened between Flensburg and Tønning in 1854.

The first railways in Denmark were built and operated by private companies. The railways in Funen and Jutland were built by Peto and Betts who also supplied the locomotives (built by Canada Works, Birkenhead). Most of the technical staff was also recruited from Britain, notably from the Eastern Counties Railway. When Peto and Betts went into insolvency, the Danish state took over Det danske Jernbane-Driftsselskab (The Danish Railway Operating Company) as of 1 September 1867 under the name De jysk-fyenske Jernbaner (the Funen and Jutland Railways), from 1874 De danske Statsbaner i Jylland og Fyn (The Danish State Railways in Jutland and Funen). The network was extended by new construction and by acquisition of the privately operated lines from Silkeborg to Herning (1 November 1879) and from Grenaa to Randers and Aarhus (1 April 1881).

The Danish state took over Det sjællandske Jernbaneselskab (the Railway Company of Zealand) on 1 January 1880, forming De sjællandske Statsbaner (the State Railways of Zealand). With the majority of railways on both sides of the Great Belt thus owned by the Danish state, it was not until 1 October 1885 that the companies of Jutland/Funen and Zealand merged into one national railway company, De danske Statsbaner (the Danish State Railways), the merger being finalised on 1 April 1893.

Important projects that followed for the Danish rail network include the Great Belt Fixed Link in 1998, the Øresund Bridge in 2000 and the Copenhagen–Ringsted Line (Denmark's first high-speed rail) in 2019.

Banedanmark is in charge of 2,045 km of railway lines, which do not include the lines controlled by private railways. All Danish railways are 1,435 mm (4 ft 8+1⁄2 in) (standard gauge), with the exception of a few narrow gauge museum railways; 1,000 mm (3 ft 3+3⁄8 in) gauge was previously common on branch lines, with 700 mm (2 ft 3+9⁄16 in) being prevalent on industry railways, such as those for transporting sugar beets or even calcium, coal, peat and many other things as well. The narrow gauge lines generally disappeared during the 1950s and 1960s.