Neilson & Co. built the first narrow gauge locomotives in 1863 for the 2′ 6″ gauge Gaekwar’s Baroda State Railway (GBSR) line from Dabhoi to Miyagram opened a year earlier. However the bullocks continued to haul the trains and the regular use of engines had to be postponed for over a decade till heavier 15kg/m rails replaced the 6.5kg/m rails. Maharaja of Baroda, the owner of GBSR, built a network of light railways connecting most of the towns in his State with the main line stations of Bombay Baroda and Central India Railway (BBCIR). Dabhoi became the centre of the narrow gauge lines and later a workshop was built at Pratapnagar, near Baroda to cater for heavy repairs of locomotives. W.G.Bagnall Ltd., Stafford, supplied the earlier GBSR locomotives whereas the carriages and wagons were built locally.

Next on the narrow gauge scene appeared the famous Darjeeling Himalayan Railway line on 2′ gauge, in 1880, which will be dealt with later along with the other two feet gauge lines.

During 1881-93, eight 2′ 6″ narrow gauge lines were introduced in the various parts of the country but none of them have survived in their original form. The year 1897 saw the opening of the first section of the 2′ 6″ gauge Barsi Light Railway (BLR). Engineered by E.R. Calthrop, this was a highly successful attempt to show the carrying capacity that could be achieved on a narrow gauge railway using high standards of equipment. BLR ordered 4-8-0T locomotives of 30-ton weight spread evenly over the six axles capable to haul almost the same trainload as a standard ‘F’ class metre gauge locomotive would do.

During the first decade of the twentieth century, several important lines were opened on 2′ 6″ gauge. The exotic 96 kilometre long Kalka-Simla line was opened in 1903 as a part of Delhi- Ambala Kalka Railway which was later made over to NorthWestern Railway. After independence this line became part of Eastern Punjab Railway till its amalgamation in the Northern Railway. This was followed by extensive narrow gauge network, built by Bengal Nagpur Railway (BNR) in the Satpura ranges popularly known as ‘Satpura Lines.’ B.N.R. also deployed larger locomotives on Barsi pattern but with separate tenders.

The narrow gauge lines were so individualistic and the standards of operation so varied that locomotive design tended to be far less standardised than on the larger gauges. Barring a few cases, most of the narrow gauge locomotive classes are limited to only one system. The locomotive standards committee on 1925 made an attempt to standardise a few classes but the attempt did not much succeed. Two classes ZB and ZE become popular, in that, large orders were placed by GBSR and BNR respectively and for other classes i.e. ZA, ZC, ZD and ZF orders remained very limited.

The classification of narrow gauge locomotives remained rather peculiar. Some locos were known by the names of the lines for which they were acquired like RD for Raipur Dhamtari line, PL for Parlakimedi line, ML for Mayurbhanj line ad ‘Delta’for Egyptian Delta Light Railway. Some locos were known by the makers’ name like Bagnall and Sentinal. Suffix `Z’ was initially used for standard locomotives suggested by Locomotive Standards Committee and has been retained for post war IRS designs as well as diesel locomotives working on 2′ 6″ gauge lines.