Superfast/ Express/Mail has same relative speed.
the first concept came with Deccan Queen Express in the past which used to run non-stop between Mumbai - Pune with only technical halts at Karjat n Lonavala.
Mail were introduced to mail service in important trunk routes.
Eg:- Golden Temple Mail, Howrah - Chennai Mail, etc
similarly...
more... express trains run at a same defined speed given to most of them. but usually gets overtaken by premier super-fasts.
take eg for Kalka Mail, which was a like express train but was up-graded to Super-Fast category later due to its premier name n run!!
Coromandel, Tamil Nadu, Kerala, Jnaneswari, Gitanjali etc are some of the most first premier super-fast trains after Rajdhani express!!
til Tamil Nadu express accident in 1998(sorry if the year maybe wrong), the RB has given the speed limit to al of the Super-fast/Mail/Express til 110kph.
but later now as proved, these trains can reach at 115kph in loose zones apart from their restricted zones.
Super-fast are given 2nd preference after the Rajdhani/Duronto/Shatabdi Express in a route. :)