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The EMD GT46PAC or WDP-4 is a passenger-hauling diesel-electric locomotive with AC electric transmission designed by General Motors Electro-Motive Division and built by both GM-EMD and under license by Diesel Locomotive Works (DLW) of Varanasi, India for Indian Railways as their classes WDP-4, WDP-4B and WDP-4D. The GT46PAC is a passenger version of the previous Indian Railways EMD GT46MAC freight locomotive. The locomotive has a 16-cylinder 710G3B diesel engine with a maximum speed of 904 RPM.
There are three variants of this loco - the original WDP-4 which is a 4,000 hp (2,980 kW) loco, the WDP-4B which is a 4,500 hp (3,360 kW)126 t (124 long tons; 139 short tons) weighted loco with a single cab and the newest WDP-4D which is a 4,500 hp (3,360 kW) loco with dual cabs.
The locomotives are numbered from Road No. 20000 upwards till 20103 and further numbering of the loco along with its WDP-4B/4D variants has started from the 40XXX series. This is due to the numbering of the 20XXX series almost breaching the start of numbering of another locomotive class, the WCG-2 which is a DC electric locomotive whose numbering started from 20105.
The WDP-4 is capable of hauling 24 coach trains at 110–130 km/h (68–81 mph). It has been also used for Spanish Talgo trial run at speed of 180km/h.
The locomotives can be used in either direction, and frequently haul trains long hood forward, as in the picture. Newer editions with 4500 hp have been produced which have been named with the sub class WDP-4B /WDP-4D which have different traction control with six traction motors and are Co-Co bogie equipped (more similar to WDG-4), unlike the original which has a Bo-1-1-Bo arrangement with four traction motors. Due to some visibility hindrances faced by loco pilots in driving long hood front, the basic WDP-4B model has been modified to dual cab to eliminate any such possibility. The other features of the WDP-4B and the WDP-4D are the same. The newer cabs have slightly wider profiles to improve visibility.
Some of the WDP-4B/WDP-4D locomotives have been spotted having a provision for an inverter-driven head-end power unit which allows trains to be run without an End-on generating car (EOG) for hotel power.
The loco is very fuel efficient and has minimal exhaust emissions unlike ALCO based locomotives, thanks to its 2 stroke high efficiency diesel engine.
The first set of 10 locos starting from road number 20000–20009 were manufactured by the Electro Motive Division (EMD) of (General Motors) while road numbers 20011 upwards have been manufactured by Diesel Locomotive Works (DLW) of Varanasi in collaboration with EMD and GM (General motors). Siemens too are part of this group providing a large component of the electronics on board this loco.
The locomotive is used on the following zones of Indian Railways:
- South Western Railway
- Southern Railway
- Central Railway
- South Central Railway
- Northern Railway
- North Western Railway
- Northeast Frontier Railway
- Konkan Railway (WDP-4 & WDP-4D of KJM, GOC, KYN and UBL are used in this zone)
- West Central Railway ( WDP-4, WDP-4B & WDP-4D of Itarsi shed and WDG-4/WDG-4D of Katni Shed are used in this zone.)
- East Central Railway
- Eastern Railway
- East Coast Railway (WDP4, WDG4, WDP4D and WDG4D of VSKP- Vishakhapatnam shed are used in this railway zone)
This locomotive is currently housed at eight sheds in the Indian Railways, spread across the country which are Hubli (Code:UBL) and Krishnarajapuram (Code:KJM) in the South Western Railway zone, Golden Rock(code:GOC) in the Southern Railway Zone, Siliguri (Code:SGUJ) in the Northeast Frontier Railway zone, Bhagat Ki Kothi (Code:BGKT) in the North Western Railway zone, Itarsi (Code:ET) in the West Central Railway, Tughlakabad (Code:TKD) of the Northern Railway, Gooty (Code:GY) in the South Central Railway, Pune (code:PUNE), Kalyan (code:KYN) in Central Railways and Izzatnagar (code:IZN) in the North Eastern Railway, Vishakhapatnam (code:VSKP) in East Coast Railways 
- WDP-4: 20005–20009, 20020,20021,20022,20026,20031, 20032, 20033, 20035 and 20065.
- WDP-4B: 40007–40009,40010,40015,40016,40018,40036,40046 and 40054–40057
- WDP-4D: 40517, 40519, 40542, 40543
The Krishnarajapuram diesel locomotive shed currently homes these locos with the following road numbers:
- WDP-4: 20010, 20019, 20023, 20025, 20027–20030, 20032, 20034, 20036–20046, 20053–20060, 20062, 20067 and 20068
- WDP-4B: 40017, 40065 and 40066.
- WDP-4D: 40226, 40228, 40236–40239, 40240, 40246, 40247, 40278, 40279, 40298, 40305, 40306, 40329, 40330, 40362, 40385, 40387, 40388, 40397, 40398, 40400, 40401, 40415, 40434, 40450, 40451, 40529, 40552
It is also currently homing the 2000th HHP High Horse Power locomotive ever, manufactured by Diesel Locomotive Works, Varanasi. The loco is numbered #40387 and is named "Pratap".
Golden Rock homes the following:
- WDP-4B: 40063,40064,40067,40068,40075,40077
- WDP-4D: 40093,40099,40100-40103,40105,40106,40114,40115,40133,40441
Erode Diesel Loco Shed homes the following WDP-4Ds. This is one of the sheds to home only WDP-4Ds WDP-4D: 40171, 40174, 40175, 40180, 40197, 40198, 40199, 40241, 40242, 40243, 40338,40339, 40488.
Siliguri houses the following locomotives:
- WDP-4: 20000–20004, 20011–20018, 20048–20052, 20061, 20063–20064, 20066, 20069-20073, 20074, 20076, 20079, 20081–20085, 20094
- WDP-4B: 20047, 20075, 40022–40025, 40028, 40029, 40032, 40035, 40037, 40045
- WDP-4D: 40089–40091,40096,40108,40120,40129,40138, 40141.
Bhagat Ki Kothi
Bhagat Ki Kothi homes
- WDP-4: 20085–20099, 20080, 20077, 20078 which are nicknamed 'Aravali' (named after the mountain ranges).
- WDP-4B: 40027, 40030, 40031, 40033, 40034, 40038–40040, 40043, 40044, 40060
- WDP-4D: 40086,40088,40092,40094,40095,40116,40127,40128
Tughlakabad (Code: TKD) homes
- WDP-4B: 40001, 40003–40006, 40011–40013, 40019–40021, 40049–40051, 40058, 40059,40079
- WDP-4D-40014,40026,40086,40087,40109,40110, 40122,40124,40125,40139
[Itarsi Junction railway station|Itarsi]] (Code: ET) shed homes:
- WDP-4B: 40041, 40042, 40047, 40048 and 40052, 40053, 40061 and 40062.
Gooty (Code:GY) shed homes:
- WDP-4D: 40104,40111,40112,40113,40117,40118,40119,40121,40130,40135,40142,40148,40150,40152,40185,40188,40314,40315,40328
Pune (code:PUNE) shed homes:
Kalyan (code:KYN) shed homes:
- WDP-4D: 40233,40234,40235"Gaurav",40260,40261,40276,40277,40312,40313,40316,40346,40347,40361,40372
There are many more sheds that home these locos and are going to home these.Gonda(NE),Izzatnagar(NE),Samastipur(EC),Kalyan(CR),Vatva(WR),RTM(WR),Erode(SR). HOWRAH, JMP (ER), Bondamunda (SE), Raipur (SECR) are the few sheds which have received this class recently and home a smaller number as of now.One by one,most or all sheds are receiving EMD locomotives of both classes,WDP-4D and WDG-4D.
The use of the locomotives "long hood forward" has been criticised because the driver might have difficulty in seeing signals. This criticism has been rejected by Indian Railways. But the solution to this problem is to upgrade all WDP-4 and WDP-4B models from single cabins to WDP-4D models of dual cabins.
Older locomotives of Indian Railways like the WDM-2 models have been upgraded from WDM-2 series to WDM-3 and WDM-3A series, to enhance other shortcomings in design of WDM-2 series.
- "Details from production Factory".
- "DLW site".
- Railway claims engine design not faulty - India News - IBNLive
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