Ammi Jaan kahti thi Railfanning se bada koi dharm nhi hota or koi Railfan chhota nhi hota - Shanzil Kabir

1) There was a Loco Reversal problem at Howrah. It reduced Journey time a lot after diversion. Earlier the bypass line didnot existed.
2)21 Platforms . 14 in sealdah North, 7 in Sealdah South
3) Platforms 1-8 are used for Local trains only in various routes like Sealdah Main, Sealdah Dankuni so those platforms can haul only 12 coach EMUs. Platforms 9,10 can hold 18 coach LHB or ICF train whereas 11-14 can hold 24 LHB trains
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Howrah , Sealdah are seperated by Hooghly River. As Kolkata is a port city constructing several Bridges will interrupt Ships which can cause downfall in Economy . But Connecting Howrah & Sealdah via Metro is in Progress.
5) Vande Bharat depot for Sealdah is under construction, Shatabdi , JS didnot start from Sealdah as Sealdah is famous for Overnight trains Mainly
6) The routes of Sealdah Division are not congested with freight traffic or Passenger traffic except Local trains . That's why Double line is enough for handling that traffic but Mumbai is different . I may not know the actual reason but any Kolkata based RF knows it better than me
Hope it helps you