Hikkaduwa Bus Station is the entrance point for most of the travellers to Hikkaduwa. There are good bus connection to Colombo, Galle & all other cities in on the way to Colombo. The Bus takes 90 to 100 Srilankan Rs. and 4 hours to reach Colombo. I would suggest you to take a Bus instead of a Train. Train was so slow and not comfortable at all.
You can reach Hikkaduwa from Galle by means of train and buses apart from private vehicle. Trains are very cheap in Sri Lanka though slow and poorly maintained. Nevertheless it's the best option to witness a bit of everyday life being among local passengers.
The trip from Galle to Hikkaduwa was a rather short ride. Bare though in mind that train stations can be very crowded as train is one of the most popular ways of locals to travel around as well.