Most travellers arrive in Sri Lanka at Bandaranaike International Airport, which is north of Colombo near Katunayake, and then want to head straight to the south coast. There is no direct route from the airport to the south. Every option involves going through or around Colombo first, and the traffic in and around the city can add significant time to your journey. Here is what each option actually costs and takes.
By taxi, direct from the airport to Weligama or Mirissa, the journey takes around two and a half to three and a half hours depending on traffic, and costs approximately $50 to $70 USD for a private car. This is the simplest option if you arrive with luggage, have an early or late flight, or are travelling with more than two people. You go straight from the arrivals hall to your guesthouse without any connections. Book in advance rather than using the official airport taxi desks, which charge more. A reputable private driver arranged through your accommodation or a local contact will typically cost less.
By train, the process is slightly more complicated but the journey itself is one of the best on the island. The train does not go from the airport. You first need to get from the airport to Colombo Fort railway station, which takes around 45 minutes to over an hour by taxi depending on traffic, and costs around 1,500 to 2,500 rupees. From Colombo Fort, trains to Weligama depart several times a day, with the first leaving around 6:50am and the last around 4pm. The journey to Weligama takes around 3 hours on the faster express trains. To Galle it takes around 2 to 2.5 hours. Train tickets in second class cost around 500 rupees. Reserved first class seats are available and worth booking in advance, especially in peak season. The coastal train route passes through fishing villages, rice paddies, and stretches of beach right at the water's edge. If you have time and no heavy luggage, take the train.
By bus, express coaches run from Colombo to Galle via the Southern Expressway in around 2 hours. Tickets cost around 700 to 1,000 rupees. Buses depart from the Makumbura bus terminal in Colombo, which is about 45 minutes from the airport. This is the cheapest option but requires the most navigation, involves the least comfort, and is not ideal with large bags.
A few things to know before you go. The expressway route is faster than the coastal road for both cars and buses. Coastal road taxis and tuk-tuks from tourist areas will often quote far higher prices than necessary, so always confirm the fare before you get in. The airport taxi desks in the arrivals area charge fixed but inflated rates. Even a short wait to contact a pre-arranged driver will usually save you money and stress.
If you land late at night, a pre-arranged taxi direct to the south coast is the only sensible option. Most buses and trains do not run at that hour, and navigating Colombo's bus system with luggage after a long flight is not a good introduction to the country.