Nizam's Guaranteed State Railway (NGSR) was a railway company operating in India from 1883 to 1950. The company began with a line built privately by the HEH, the Nizam, which was owned and operated by the company under a guarantee from the Hyderabad State. Capital for the line was raised by issuing redeemable mortgage debentures. The Nizam's railway was eventually consolidated with the Hyderabad-Godavari Valley Railway (HGVR). In 1951, both the NGSR and the HGVR were nationalised and merged into Indian Railways.

The GIPR line connecting Bombay with Madras had bypassed the Nizam's territories. The Britishers were interested in joining the GIPR line with Hyderabad and make the Nizam incur all the expenses and pay a guaranteed interest to GIPR. On 19 May 1870 an agreement was signed by Governor General Lord Mayo with the Nizams. As per this agreement, the new company would be owned by the Nizams and be known as Nizam's State Railway. The capital to set it up would be provided by the Nizam, but will be constructed and operated by Government of India through British Resident at Hyderabad. Hyderabad was connected to GIPR and a new line from Wadi to Secunderabad was finalised. The construction of 192 km (119 mi) line started on 25 March 1871 and was completed on 9 October 1874. The line was split at Begumpet with one going to Secunderabad and another line going to Hyderabad. The 140 kilometres (87 mi) line from Secunderabad to Warangal was opened on 8 April 1886. Later, starting from 1889, metre-gauge lines were laid from Manmad Junction railway station to Secunderabad, connecting Aurangabad, Jalna, Nanded and Nizamabad. Financial deterioration of NSR coupled with the need to extend the railways to Singareni to carry coal to the GIPR forced Salar Jung to seek funds in the London financial markets. Nizam State Railways was taken over by Morton, Rose & Co, a joint stock company based in London and renamed as Nizam's Guaranteed State Railway on 27 December 1883.

Nizam's Guaranteed State Railway was formed on 27 December 1883 after the Nizam State Railways was taken over by Morton, Rose & Co. As per the agreement signed the Nizam's government had to deliver all existing railway lines free of all encumbrances to the newly formed company. The Nizams had to pay annuity for 20 years to the newly formed company. This company had ten directors out of whom only one was an Indian. The lone Indian member was Sardar Diler Jung Bahadur who was the Secretary of the Railways Department of Nizam's Government. On 1 April 1930 the NGSR was brought under the direct control of Hyderabad State with Sir Akbar Hydari as the president. The remaining members of the board of the nationalised company were Britishers and the headquarters of H. H. Nizam's State Railways remained at London. The office was relocated to Hyderabad on 1 November 1941, as the ongoing war was making it difficult to work over such a long distance. Hyderabad was integrated with India in 1948 and on 5 November 1951 NGSR, along with GIPR and some other small railway companies were merged to form Central Railways. On 2 October 1966, South Central Railway was carved out of Central Railway with areas of erstwhile NGSR under its jurisdiction and Kacheguda as its headquarters. At the time of merger, the total length of Nizam State Railway system was 2,351 kilometres (1,461 mi) and was the largest system in any princely State in India.

The proposal was for an initial railway line to be built from Secunderabad Railway Station in Hyderabad to Wadi Junction. Nizam agreed to fund the construction expenses for the initial line, leaving subsequent branches to be financed through a variety of means. Construction commenced in 1870, and the Secunderabad-Wadi Line was completed in 1874. Between 1874 and 1889, this line was extended to Kazipet and then to Vijayawada.

In 1879, the Nizam Mahbub Ali Khan, Asaf Jah VI took direct control of the company, integrating it into the state bureaucracy as the state-owned Nizam's Guaranteed State Railway. This partial-nationalisation was reversed in 1883 when a management company was formed to gradually take over the lines, under the provision of a guarantee from the government of HEH, the Nizam of Hyderabad State.

In 1899, the broad gauge connection between Bezwada (Vijayawada) and Madras (Chennai Central) opened, making rail travel between Hyderabad and Chennai possible. Railroad tracks in the state thus contained 467 miles (752 km) on the broad gauge, all built before 1891, and 391 miles (629 km) on the metre gauge, which were opened between 1899 and 1901. The total capital expenditure on the Nizam's State Railway at the end of 1904 was 4.3 crores. In that year, the net earnings were nearly 28 lakhs, or about 61⁄2 percent of the outlay.

In 1916, another railway terminus, Kachiguda Railway Station, was built to serve as the railway's headquarters. The Nizam's railway was then divided into various, directly owned subcorporations. Each had a head official appointed by the Nizam's Railway. The profits of these rail lines were distributed by the Nizam's Railway.

The Hyderabad-Godavari Valley Railway was a 1,000 mm (3 ft 3+3⁄8 in) gauge railway. John Wallace Pringle — who had recently completed surveying routes for the Uganda railway — was appointed as the superintending engineer in 1896. The railway opened in 1896, with a 391 miles (629 km) line from Hyderabad city to Manmad Junction. The railway eventually grew to 467 miles (752 km) of 5 ft 6 in (1,676 mm) gauge track and 391 miles (629 km) of 1,000 mm (3 ft 3+3⁄8 in) gauge track. The Hyderabad-Godavari Valley Railways cost 2.6 crores, and earned 7.7 lakhs net in the same year, or nearly 3 percent. In 1901 and 1902 the earnings were about 31⁄2 percent.