Private buses are the first option for most of the people staying in Bangalore to travel to Kerala. For me too it was the same till a couple of months ago. But their arrogant and irresponsible behaviour and over charging on weekends and special days made me to prefer trains recently.
The gareeb rath train runs on alternate days from Bangalore is a good option. The timing is convenient, train is neat and easy to get tickets too.
The picture below shows Hosur railway station where the train stops and is a preferable stop for people from electronic city area.