A new passenger rail service linking northeastern Thailand and Laos across an Australian-funded friendship bridge over the Mekong River has been officially opened.

The rail line runs from the Thai town of Nong Khai, 600km from Bangkok to Tha Naleng on the Laos side, just 20km from the capital Vientiane.

The Australian-funded $A42 million 1.17 km bridge, first opened in 1994, has been a highlight of Australian diplomatic efforts in assistance and cooperation.

The opening of the bridge's rail service early on Thursday was presided over by Thai Princess Maha Chakri Sirindhorn, as well as Thai Prime Minister Abhisit Vejjajiva and Lao Deputy Prime Minister Boonyoung Worlajit.

Australian diplomats from Thailand and Laos were also present.

The bridge has been seen as a symbol of regional trade and economic integration, adding to the growing regional transport links across Laos, Cambodia and Vietnam and extending into southern China.

The bridge is also a key part of the region's transport development that extends road links from Singapore through to China's port city of Shanghai as part of the Asian Highway 12.

State Railway of Thailand officials expect about 500 passengers to use the 3.5km rail service daily.

The train has a schedule of four round trips a day over the Mekong River. A third-class ticket costs 20 baht (90 Australian cents) and a second-class ticket is 80 baht ($A3.50).

Reports said Thailand and Laos had agreed to extend the rail service from Thanaleng to Vientiane in the future.