That would take a long explanation. Try to understand.
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We shall talk about Goods Locomotives and in this topic, I shall speak only about Howrah & Asansol division. There is a policy of running Diesel & Electric Locos in above division, we shall see them. I am not a railway official or power controller, I will just explain in a Railfan's point of view & observation.
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have a huge fleet of Electric Locos as well as Diesel Locos.
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Generally, Electric Locos run mainly on route •Dumdum/Kharagpur-Dankuni-Barddhaman-Asansol ( /blog/post/4604833 )... U will find most of the freights on this line with Electric locos. So Railways mainly utilize Electric Loco on the above route. Though u will find some freights on Sahebganj Loop with Electric Loco.
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Diesel Locos run on the following routes
•Andal-Siuri-Rampurhat & further •Andal-Khana Link cabin-Rampurhat & further • Barddhaman yard-Rampurhat & further ( /blog/post/4605083 ,
/blog/post/4605072 ) •Barddhaman-Andal (sometimes) . Apart from these, u will find freight rakes (atleast about 1 per day) on •Kharagpur-Barddhaman-Rampurhat & further, running with KGP ALCo, VSKP ALCo/EMD (I even found R,KJM,GTL,PUNE ALCo/EMD). So these ALCo/EMDs (mainly of South sheds) running on KGP-BWN-RPH, are just running to join duty on NFR.
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Now understand what I am saying, there are still enough diesel locos on IR. But where will these ALCo s of KGP,BNDM,BKSC,VSKP work? Most of their zones are electrified. So these sheds sent their ALCo/EMD to NFR/ER for duty. And so some freights on KGP-BWN work with ALCo just to bring the links to BWN, and from there they will be sent to NFR. Hence u will find huge number of KGP ALCo in NFR/Sahebganj Loop. In this picture, u see a BKSC ALCo. Now, BKSC & BNDM DLS sent their locos via-ADRA-Mohisila-UDL whereas KGP & VSKP DLS sent their Locos via-KGP-ADL-BWN. ( /blog/post/4604655 )
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Again, it doesn't mean that electric locos don't run in Sahebganj Loop. They run, but less compared to Diesel locos. Just as an example, the Sagardighi power plant receives Coal rakes (2-3/day) from Andal and Railways electrified the RPH-NHT-PRDG-MGLE so that the coal rakes can run with Electric Loco. But every day, Diesel locos take freights to the power plant! So there is more Diesels being used. ( /blog/post/4604785 )
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Now u will need to understand the policy of Barddhaman yard. They have a smart policy and u shall understand it if u spent some time there tracking the rakes & locomotives. I will try to explain here... Barddhaman is simply a LOCO CHANGING POINT. Freights from Dankuni to Asansol side, mostly NEVER have loco change at BWN. Whereas, freights from Dankuni to Rampurhat, usually HAS Loco change at BWN (E/Loco to D/Loco). Similarly, Freights from ASN to DKAE never have L/C at BWN. Whereas Freights from RPH to DKAE usually has L/C at BWN (D/Loco to E/Loco), Bcoz the freights coming from RPH side come with Diesel power.
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Again a small policy of BWN (I don't know why they do that)... Freights coming on KGP-ADL-BWN with D/Loco (fresh Locos from DLS to join duty in ER/NFR) come with the Diesel Locos as "active" (i.e,they do NOT come idle or shut down behind any E/Loco) ... But, when the same fleet of D/Locos are required to sent back to their DLS in South, they are sent idle or dead behind an E/Loco hauled freight. ( /blog/post/4604254 , /blog/post/4589722 )
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Again u may find some HWH ALCo running with Freight between DKAE-BWN ( /blog/post/4604832 ), but u will rarely find a BWN ALCo or UDL ALCo doing that. So shed also has some running policies.
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Even sometimes MLDT/NGC sends some ALCo /EMD to KGP for POH & general duty. So u will find some of them working there & coming back. ( /blog/post/4582179 )
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So that was the freight running policies followed here. All Locos are being properly utilized, but mostly on dedicated routes (maybe due to availability of crew)