When I said you are wrong, but in this case most people need overnight train and look at the train list and availability of train from Chennai-Coimbatore and vice versa. From Coimbatore during night time there is a train to Chennai every 15-30 minutes. During peak time like Pongal, Deepawali you won't even get a seat in Intercity Express which have 1300 availability. Why a flight like IndiGo which is the only profit making airline operate Chennai-Coimbatore atleast 3-4 times a day. Because people prefer flights also on this route when trains are not available. Sometimes cost of flight is cheaper than the buses during festivals and holidays.