KGP-CTC stretch stations have BBS and KGP for catching trains for Mumbai. They can easily travel to BBS or KGP to catch Mumbai trains. No need to run a train from HWH/SRC to CSMT/LTT via a longest route when shortest route is available via TATA. But yes if ECOR wants can run a train from BBS to CSMT via BLS, HIJ, TATA. But still i don't feel it is much needed now, when connectivity with BBS or KGP is available for passengers to catch train.
Many stations don't have direct connectivity to many stations if u see even KGP don't have a direct connectivity till date to Amritsar, passengers go to HWH/SDAH or TATA to catch ASR train. Although talks r on...
more... for a KGP-ASR train but let's see if that happens. So, it is not necessary that every station will have direct connectivity to every other stations in IR.