You have got all the points what I have mentioned today morning in my blog 3. The purpose of bypass line is to use in case of emergency when they is an accident in the section from Chennai to Salem the south bound TN trains can be diverted and avoid total cancellation of all trains. There is no use of running trains or EMU from Central to MS as you mentioned it takes 30 mins while u can get within 15mins by walk hardly 1km by road The same applies to express trains.