We took the Niligiri express for our Ooty honeymoon trip and I would say that it was a very wonderful experience. I have travelled in other south india based trains and know of the standard that is maintained in everything. This train is very clean. This was started when Britishers wanted to travel to Ooty from chennai during Summer. So they still maintain the punctuality and cleanliness tradition.
I read an another review on this by a friend who had a bitter experience. I have not travelled in AC and cant comment much on that. Also I have travelled a couple of times and completely satisfied with the service for the money spent.
I reserved the first class 2 seater cabin and we had a wonderful time watching the stars/moon and admiring nature. You would lose the charm of nature if you would have boked a AC ticket..I enjoyed it even more than the actual stay in Ooty – without exaggeration!!