The combination of Kandy or the cultural triangle with Sri Lanka's south coast is one of the most common trip structures for foreign visitors. The connection between the two is more straightforward than it looks on a map, but it does take half a day either way. Here is what each option actually involves.
By train via Colombo. The most common route is Kandy to Colombo by train and then Colombo south along the coast. Kandy to Colombo on the intercity express takes around 2.5 to 3 hours and costs LKR 200 to 500 depending on class. From Colombo Fort station you connect to the coastal line heading south. The train to Galle takes another 2 to 2.5 hours and this coastal stretch is one of the more scenic train rides in Asia, running right along the water for much of it. Total journey Kandy to Galle by train with the Colombo change is 5 to 6 hours. Budget around LKR 500 to 800 all in.
By private driver. Kandy to Weligama or Mirissa direct by car is roughly 3.5 to 4.5 hours depending on traffic and the route. A private driver for this transfer costs around USD 50 to 70. For three or four people splitting the cost this is often better value than the train, door to door with no connection to manage. Make sure to agree the price before you leave and confirm whether toll fees are included separately. Most drivers will quote a price that excludes tolls.
By bus. The cheapest option. Kandy to Colombo by express bus takes about 2.5 hours and costs LKR 150 to 250. From Colombo you pick up a bus heading south toward Galle, Matara and beyond. It is slower overall and more effort with luggage changes, but if budget is the priority it works. Buses from Colombo to Galle run frequently and the journey takes around 2 to 2.5 hours.
From Sigiriya or Dambulla. If you are coming directly from the cultural triangle rather than Kandy, the drive south is longer. Sigiriya to the south coast direct is around 5 to 6 hours by car. Most people either stop in Kandy for a night to break the journey or take the train via Colombo which adds a change but can be more comfortable.
Which towns to head for. Galle is the first major stop on the south coast coming from the north and a natural place to settle if you want to orient yourself and explore from one base. If you already know you want beach and surf, go straight to Weligama, Mirissa or Hiriketiya rather than stopping in Galle and moving again a day later. The towns are close enough that you do not lose much by going further on the first day.
A practical note on timing. Colombo traffic is genuinely bad during morning rush hour, roughly 7am to 9:30am, and evening rush, 5pm to 8pm. If your route involves driving through Colombo, either go early or wait until mid morning to leave Kandy. The time difference between leaving at 8am and leaving at 10am through Colombo can be 45 minutes to an hour.