In 1971 I traveled the length and breadth of India all by rail, 3rd Class Unreserved.
Cheap -- absolutely. A good thing since my finances were, shall we say, tight. Many, many 100s of miles for very few $.
Comfortable -- not so much. Getting a seat was almost beyond hope. Claiming a luggage rack was a conquest. My last ride was overnight to Bombay as it was known then. I slept, sort of, standing packed so tight one couldn't fall over.
Accessibility -- sure, but one had to be fast. Forget boarding by platform. Go for the window and just crawl in.
Sociology -- a good way to get to know the locals though one had to be careful when in the luggage racks not to show anyone the bottoms of your feet, a grave insult.
Memorable -- you bet.