See, let me explain this. I plotted this in concept of Day express.
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As per your idea to depart from TBM at 06:45. If some one has to board this train at TBM terminus they have to take an EMU from egmore / park station at 6 am itself, how much morning trains arrive within say 05:45 morning at central? Also how come people from tiruvallur/avadi could reach TBM within 06:45? Atleast they have buses but still bus takes 1h time and they need to start 05:30 at home which is odd....
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But for gummidipoondi that north chennai area they even dont have proper EMU connections, even at that wee hours, how do you expect them to take train at 06:45 hrs in Tambaram?
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Ok lets see the list of trains arriving at chennai central in morning. I am mentioning this because i hope you are atleast aware of tourist influx to rameswaram.
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13352/18189, 11027, 12839 and some weeklies like MFP YPR, SHM TVC (that slot sharers which touches PER/MAS) are the only trains which arrive central within morning 5 am (not taking trains from ED/SBC end as it is ridiculous for them to change at chennai at RMM). Even 12604 comes at 05:55 hrs which will be difficult for them to catch this train if it departs at 06:45 from TBM..
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If i depart by 9 at MS itself, then the above trains along with Grand trunk, tamilnadu SF, circar from Godavari region of AP, Charminar and KCG CGL may also be connected, facilitating better connections to rameswaram from NCT and north.
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Now you may ask 12604, 17652 and 12760 are trains from HYB right what if only 12604 is connected. For your kind information all three trains take three different routes. 12604 covers guntur, 12760 covers warangal, 17652 covering kurnool respectively.
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Hope you can understand atleast now why i choosed this timings. Operational constraints + connecting more morning arrl trains from north to central + to facilitate easy connection to north and west part of chennai.