दौड़ती है हिमालय तट से सागर, हमसफ़र हर राह पर, हवाई जहाज़ों को त्याग कर, करो अक्सर, भारतीय रेल में सफर. - Arpit P

I think after electrification of Howrah/Sealdh - New Jalpaiguri route all train be it passenger or goods will get a electric loco up to New Jalpaiguri. Well as for the Railway Electrification work is going on in this section it will take another 2-3 years.
Loco change can be done at New Jalpaiguri for the above mentioned trains by you. I do not think for 560 KM a train will get Diesel Locomotive where Electrification is available.
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Never...
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mind i think its high time to convert this route suitable for electric loco. WDP are superb engines but they are only 4500 HP.
A WAP 7 electric loco is 6520 HP so the difference comes out like 6520 - 4500 = 2020 HP... I still wonder so see very slow progress of railway electrification work in NFR region... Though it was sanctioned in the year 2008.
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TO speed UP Darjeeling Mail we need P7 and good track... Same goes for good train we need WAG 7 and WAG 9... Or give twin WDG for goods. I hope their is not enough WDP/WDG to cater all trains in NFR & East in Siliguri Shed...