Recently stayed in Retiring room (Feb 2019) in DMM railway station - attached are the pictures of it.
Minimum booking period: 24 hours
Check-in / Check-out: 6:00 AM
Charge per day: 300 Rs (5% discount for online-bookings)
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rooms avbl in Stn: 2 rooms (Double-bed, Non-AC)
Overall, a decent stay. The inside of the buckets in the bathroom could be a bit neater, rest everything is fine. The windows in the room have a mosquito mesh as well, so you could have fresh air. However, the ventilation window of bathroom is not covered, so mosquitoes could enter from there, so i recommend you carry your own repellent.
Though, there is an option of booking the retiring room on-the-spot, in the station itslef, currently (on my visit in Feb2019) the computer which allows them to do so, (which shows them the live availability status) is not functioning - so the railway station staff are forced to request the passengers to go to a nearby internet center & book the room online itslef. To avoid this possible hassle/time-wastage, i recommend you to book the room online beforehand (+ paper-printout of the retiring room booking voucher is mandatory) & you may just show the ID card you're carrying.
Additional obesrvation: outside the retiring room, on the platform, there is a spot where it's crowded with trees and is very peaceful. They have few benches there (beneath the tree cover) - you could just sit & relax there peacefully. [Most of the trains arrive on platform 1 and platform 5] - so the platform 2/3 where the retiring rooms are present - is very quiet & hardly any people at all

This train (MDU RMM) is kept on the Madurai platform way before the departure time of 12:40, so if you come early (~1 hour before departure), you can get a nice window seat, and try getting a seat to your left (direction MDU RMM) to avoid the sun. The train will be quite packed by the time it departs Madurai, and majority of the passengers get down at station PMK. And after station RMD, this train runs quite empty.
Once the train approaches the final phase of it's journey (way before the pamban bridge), you can spot the sea far away on both the sides of the train, and as the train progresses, you can notice the sea shore...
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getting closer and closer towards you. Just before the train enters the bridge, you can spot many peacocks on the side of the tracks. Once you enter the bridge, (track speed limit: 15 kmph), if you look down, you can notice the sea water gushing with pace in a certain direction, and if you focus fully down, it's almost scary / thrilling at the same time ... makes you wonder how in the world did they construct the bridge in the middle of sea. You also have a wonderful view of sea all around. Even after you cross the bridge, before arriving at RMM, the sorroundings would be very pleasant with white beach sand dunes/palm trees/coconut trees... See the attached pics. You almost feel like you're strolling on a beach, but you're on a train.