Overall analysis:
Cleanliness: Despite old coaches , 12636 is very neatly maintained by MDU division. But in 12635 cleanliness may get a little bit down as it is 4th run without PM for the rake. But still 12635 is also better than many trains in cleanliness.
Punctuality: Mostly OT or BT. Sometimes may get a little delayed especially between TPJ-MS.
Food: Before the train...
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departs itself , the catering starts. On-board catering is excellent both ways. Lot of choices at reasonable costs with nice taste. Tea is sold near TPJ. Manaparai Murukku sold by vendors at DG is a must.
Ticket availability: Tickets are easily available in 2S class. You can even get in current availabilty. But this does not apply for weekends and season time. AC class is a little not so easy and hence you need to book your tickets in advance.
U/R coach: This gets a little crowded and many of them are end-to-end travellers.
Railfanning: It's a railfan's paradise. Many crossings are visible till TPJ. After that less crossings till MS but a superb-high speed run is there.
Safety: Though no RPF/GRP are there , people are decently behaved but be a little careful of your seats , especially near VRI
Rating 9.5/10.