Yes, you are right your view and agree.even i checked list of train to South. It is very few when i wanted to reach RMM from madurai.
However Island Exp is considered to be train for Keralites.
Even for Coimbatore, No dedicated Train from Bangalore. Island is 2nd Option next to Kurla if seats are unavailable.
If i am right, there are...
more... some trains from CBE/ED to South TN (apart from 16609/10) such as Nagercoil Exp (16339), NCJ Passenger , TEN passenger train,Mumbai CST-Nagercoil/TEN.