Generally, Indian Railways are very efficient, but Indian trains do run late, and sometimes its hours rather than minutes. To get a feel for it, why not go to ERAIL and see how late yesterday's Delhi-Jaisalmer Express arrived or last Thursday's Bombay-Delhi Rajdhani Express? At ERAIL, select the origin and destination that interests you, and bring up the train list. Now find the train that you want and click on it.  Now select a date and click the 'train running status' button. It will show you a table of scheduled times and actual times at each station. Data is only held for the last few days, not weeks or months ago.

Alternatively, these examples from my own travels may give you a feel for the likely delay:

Delhi-Varanasi overnight express spot on time, Bombay-Calcutta Mail 1½ hours late, Madras-Bombay 'Chennai Express' 40 minutes late, Calcutta-Delhi 'Rajdhani Express' spot on time (Rajdhani Expresses get priority and are pretty punctual), Delhi-Agra 'Shatabdi Express' spot on time (Shatabdi Expresses also get priority and are pretty punctual), Jaisalmer-Delhi Express 2 hours late starting and 3 hours late arriving, Delhi-Kalka-Simla 'Himalayan Queen' spot on time, Varanasi-Agra-Jaipur 'Marudhar Express' 50 minutes late, Delhi-Madras 'Grand Trunk Express' 1½ hours late.

Other Indian train tips:

Bring your own toilet paper. You'll normally find one one western toilet and one squat toilet at one or both ends of the car. In AC1, AC2, AC Chair Class and even AC3 the toilets are normally reasonably clean by Indian standards, and in full working order.  Sleeper Class and 2nd class toilets may be a different matter..! South India Travel