I think that IR was supposed to construct an Electric Loco Shed at Dhanbad but due to unavailability of land, they shifted their location to NSCB Gomoh Jn.
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A Trip Shed is required as it does the job of a "Rain Basera" or Shelter for visiting locos for a night or two. The Home Sheds on the other hands are already packed...
more... with their own locos and act as garages for their maintenance, quick-fix checks, etc. apart from just homing them. Mughalsarai has a dedicated ELS for homing it's WAP-4s and WAM-4s while it's Trip Shed homes WAP-5s and P-7s from Ghaziabad, Gomoh and Howrah.
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I remember that IRFCA Veteran Member Samit Sir had once told me that Loco Sheds serve routes and not towns. Now Ghaziabad has an ELS that homes electric locos in the Delhi Area while DLI and NDLS have the trip sheds homing GZB and LDH Locos (Both electric and diesel). Now Howrah needs a dedicated ELS/DLS for providing locos to originating ER trains asap. In the same way SRC requires to serve SER trains beginning from HWH New Complex or SRC/SHM :-)