the best thing for you is not to book in any train. You can buy ticket once you are at Bangalore railway station.between Bangalore and Mysore you will find lot of seats specially in upper class. Very rarely people book ticket. Most of them buy ticket at station and travel. Also if you have lots of laguage and of you are flying to Bangalore then it is better to take a cab to Mysore than catching a train. It takes around 3.5 hrs from airport to Mysore on cab.