Just nowdays, dont ask for 55 KMPH for superfast categories, but just compare other existing trains on the route!!!
If a train has marginally faster run, compared to regular express train, in that route, IR will term it as Superfast!!
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(U may find many trains with marginally better or same run, but the slower is superfast & faster is Normal Express - Just...
more... search in between MGS-BSL, find Shramik (Non-Sup), Tapti-Ganga/Bhagalpur-LTT Express (Sup), while Shramik is fastest till Surat (compared with Tapti Ganga), while Bhagalpur is slowest!!!)
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GR in some route compete with Raj (Ranchi/Chennai), while somewhere trail behind Sup (BSB GR vs Shramjeevi/Shiv-Ganga)
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DRNTO in some route are faster then Raj, while somewhere are trailing behind (ERS) & where no Raj exists, they compete with Sup & are too slower then GR in some other routes (HWH-YPR)
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So, its all about route, if u r fastest in that route (no matter below 55 KMPH) u r Sup!!!