Rockfort Express: திருச்சி அவனது கோட்டை, இரவில் சென்னை வரை வேட்டை, அவன் தான் மலைக்கோட்டை. - Vijay Baradwaj

If you are registered with IRCTC then log in
then
2- Look for tab "Tourism"
3- It has Accommodations Tab which opens two links Hotels and Retiring rooms
4- clicking Retiring Rooms will...
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get you a page you will have to again Log in
5- after log in you get the page that enables you to fill up form starting with PNR and then follow the easy steps of booking at origin or destination
Try once and you get to know how it is done

Lounge is a place where you are relaxing in a comfortable probably AC room with cushioned chairs / sofas. and has TV charging points, bath toilets and is available on hourly basis.
First time I tasted this was at MAS courtesy our friend Pramodh K who is also a longtime on this forum.
The dormitories are cluster of single beds in big hall with lockers and common bath toilets fans. Both types are available AC or Non AC
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upon grade of stations the furniture and layout is different.
There are Male female dormitories as well.
Then rooms which are exclusive to occupant lockable with all amenities as are available in a modest hotels Toilet Bath Geyser
Single rooms and Double rooms Ordinary are available
Deluxe rooms have TV added
and Suits (may) have carpets and all British heritage visitors Sofas and may be they drop local news papers. But I have not stayed ever in suits as alternative accommodations are usually available if one books in advance.
Rooms are good for family.
Online booking helps avoid all doubts and misunderstanding.

Usually your booked room/dormatory is not allotted to anybody even if you are late in occupying for the reasons of Train Late.
But if Desk officials play foul then you can lodge a written complaint which they Fear and will never irritate you. So do not worry. If you are late and the room is allotted to someone, then insist for alternative accommodation and ask for complaint book. and raise the issue with higher officer. In this case higher officer is Station Master. And still you are grieved then do not hesitate to ask for GRP help.
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after your tour is over, drop a letter of your experience with irctc and on Govt portal www.pgportal.gov.in > Railway Board with all the relevant information references of your ticket and booking ID Room no. name of officials station name etc etc. In Short Story should be Long. And this is not to propagate the Fighting with Govt but future incidents can be avoided.