On reaching the railway station, you find that the train you wanted to catch is just to start and there is hardly any time for purchasing the ticket. The best thing for you is to :

rush to the train rather than miss it and inform the T.T.I. at the next stoppage about your inability to purchase the ticket.

rush to the train and perform your journey quietly.

first purchase the ticket and then catch the train if it is there.

miss the train rather than take the risk of boarding the moving train.