Hussain Sagar Lake distinguishes both the cities apart, Both of them have separate Lok Sabha constituencies but yes you are right as Secundarabad comes in Hyderabad District and Mainly more train movement is there, Secunderabad can be considered as the Biggest station of Hyderabad. AFA kachiguda is concerned I don't why people like it That much, It seems ordinary to me except pf no 1 all platform are lacking facilities.