Its widely spoken but I found profiency varied greatly. The middle class Indians that use AC2 and AC3 are far more likely to speak English to a good conversational level. Personally I wouldn't use Sleeper or second class, I am not 18 and on a gap year!
I used second class because getting off at Agra was unplanned, I hadn't realised the train stopped there, so when I saw it did I just grabbed my bag and jumped off - and needed a way onwards to Delhi after visiting the Taj and Red Fort. One question that was asked of me by the rather inquisitive people I found in there was why I (as a presumably rich Westerner) didn't just bribe the guard to get into a reserved coach.
Sleeper Class was planned, and it was the kind of adventure I was looking for for my £8.50...though I was a bit younger then, I was in my late 20s/early 30s, not 18!

The heat (and it was hot, it was June) wasn't much of an issue as all the windows and doors were open so there was a pleasant breeze.