Indian locomotive class WAM-4
The Indian locomotive class WAM-4 is a class of 25 kV AC electric locomotives that was developed in 1970 by Chittaranjan Locomotive Works for Indian Railways. The model name stands for broad gauge (W), alternating current (A), mixed traffic (M) engine, 4th generation (4). They entered service in March 1971. A total of 500 WAM-4 were built at CLW between 1970 and 1983, which made them the most numerous class of mainline electric locomotive till its successor the WAG-5. The WAM-4 is one of the most successful locomotives of Indian Railways having served both passenger and  freight trains for over 51 years. This class provided the basic design for a number of other locomotives like WCAM-1, WAG-5A, WCG-2, and some WAP-1 models. However, with the advent of new 3-phase locomotives like WAP-5 and WAP-7, the WAM-4 locomotives were relegated to hauling smaller passenger trains and now the aging fleet of WAM-4 locomotives are being rapidly scrapped and have been fully withdrawn from mainline duties. History. Origins. In the 1970s, the Indian Railways started a series of study projects for a high horsepower locomotive. Although the WAG 1/WAG-4 was being introduced, officials believed that a  much larger and more capable locomotive was needed, especially to haul Freight trains that was heavy for any existing locomotives. These studies led to initial requirements for an locomotive with a tractive effort of and a speed of .The WAM-4 was conceived while goods traffic was increasing in the 1970s. The era of Co-Co locomotives, led by the enormous popularity of the WDM-2, had revolutionized long-distance travel. With the aim of addressing the shortcomings of the previous WAM-1/2 and WAG 1, WAG-2, WAG-3, WAG-4 classes and remove steam locomotives from IR by a target date of 1990. The  WAM-1s were not great successes as some of their advanced features were unsuitable for Indian conditions. so the designers at RDSO and CLW decided to use the following Production. Production of these locomotives started in 1970 with #20400 and exactly 500 WAM-4s were built in 13 years with #21399 “Anant” being the last one. Production ended on August 3, 1983.WAM-4 though an indigenously produced loco was heavily inspired by French box design and naturally favoured a French TM which was the Alstom made TAO TM. A single WAM-4 can generally haul up to a 24-coach passenger rake. This class  proved so successful by virtue of its ruggedness suitable for Indian conditions and simplicity of maintenance. In January 2020, the last WAM-4 units (four from Tata ELS) were withdrawn from mainline service. Currently 8 locomotive are currently doing shunting and departmental works. Rebuilding. A number of locomotives were rebuilt as WAM-4B in the late 1970s to evaluate Alstom TAO 659 Traction Motors and for exclusive use on freight duties. These 'locos' were ballasted to improve traction and had excellent load-hauling capabilities, This experiment gave rise to the future WAG-5 locos. Duties. The class was designed for both passenger and  freight work. Many of the original locomotives were fitted with vacuum brakes only. With the withdrawal of many WAG-1 and WAM-1 locomotives in the 1990s, the WAM-4 units were given air brakes to prolong their life into the 1990s and beyond. Sub-classes. This loco class has been seen in many variations, as a lot of workshops and sheds have carried out their own enhancements or modifications to the basic loco design. Although the code indicates a mixed-use loco, most WAM-4's ended up hauling passenger trains. Variants include The 'DB' or 'D' generally, but perhaps not always, indicates  dual-brake capability. 'HS' may be for 'high speed'.'2S', '3P', '6P', etc. indicate traction motors connected in series or parallel. The WAM-4 has six traction motors, and originally they were wired to be available in different configurations at different power settings. At notches up to 14, all motors were in series (at notch 14 all resistors dropping out); up to notch 21 in series-parallel combinations (three pairs of motors in series, the pairs themselves being in parallel); and further notches with all motors in parallel (at notch 30 all motors are in parallel with resistors dropping out). This  is the original configuration of the WCAM-1 series of locomotives too. The WAM-4 locomotives were later reconfigured to have all motors always in parallel (6P variants) or with the three series-connected pairs in parallel (2S 3P variants). Some WAM-4 locomotives from CLW are thought to have had the 2S 3P configuration right from the start. The 2S 3P configuration was better for the mixed traffic loads especially as it allowed the locomotives to start hauling larger loads without stalling. With increasing use of the WAM-4 locomotives for passenger traffic the all-parallel configuration was deemed more desirable since it  allowed higher speeds and higher acceleration. Livery. This loco has the widest variety of liveries, with each loco shed having its own livery Most of the WAM-4 locomotives now have their MU capability disabled as RDSO disapproves of these locomotives running MU'd over 100 km/h. Named locomotives. A few WAM-4 locomotives have been named by Indian Railways Preserved Examples. As increasing numbers of WAM-4 have been retired, some have found their way into museums or other uses. A few WAM-4 locomotives have been preserved by Indian Railways at various location around India