How were the 9 older zones created out of the former independent railway networks?

Around 1950, legislation was passed allowing the central government to take over many of the independent railway systems that were in operation.

In 1951, the following zones were created:

SR — April 14: From Madras & Southern Mahratta Rly., South Indian Rly., and Mysore State Rly. (about 9660 km). CR — Nov. 5: From the GIPR, the Nizam's State Rly., and the Scindia and Dholpur Rly. (about 8690 km). WR — Nov. 5: From the BB&CI Rly., the Saurashtra, the Rajasthan, the Jaipur, and the Cutch Rlys. (about 9120 km). In 1952, the following zones were created:

NR — April 14: From the Jodhpur Rly., Bikaner Rly., three divisions of the East Indian Rly. north-west of Mughalsarai, and the Eastern Punjab Rly. (about 9670 km). ER — April 14: From the rest of the East Indian Rly. (east of Mughalsarai) and the Bengal Nagpur Rly. (about 9120 km). NER — April 14: From the Oudh-Tirhut Rly., the Assam Rly., and the Kanpur-Achnera section of the BB&CI Rly. (about 7660 km). In 1955, the South-Eastern Railway is carved out of ER:

SER — August 1: ER retains three divisions of the old East Indian Rly. and the Sealdah division of the old Bengal-Assam Rly. (about 3740km); the rest becomes SER (about 5380km). In 1958, the Northeast Frontier Railway was carved out of NER:

NFR — Jan. 15: the former Assam Rly. lines in the Pandu region are split off from NER to become the NFR (about 3730km). In 1966, South-Central Rly. was carved out of SR:

SCR — Oct. 2: The former Nizam's State Rly. network (by then the Secunderabad and Solapur divisions of SR) and part of the former Madras & Southern Mahratta Rly. (by then the Vijayawada and Hubli divisions) are split off from SR to become the South-Central Rly. (about 6070km). Adjustments: Later in 1977, Guntakal division from SR was merged into SCR, and the Solapur division transferred back to CR. There have been other minor adjustments; e.g., in 1988 Ruthiyai-Bina was transferred from WR to CR. Akola-Purna was transferred from CR to SCR in the late 1970s. Malda was moved from NFR and made part of ER in the early 1980s. In 1979, the Shoranur-Cochin section was transferred from the Palghat division (originally Olavakkot -- included Cochin - Shoranur and Jolarpettai - Shoranur) to the newly formed Trivandrum division. Minor adjustments including transfers of portions of various routes from one division to another within a zone are quite common.

Source - IFRCA.org