Laos officials say the new rail link will help cut export costs on goods
Thai Princess Maha Chakri Sirindhorn has opened a rail link over the Mekong river, linking Thailand and Laos.
Two passenger trains will run the 30-minute return trip each day, serving about 500 people daily.
The new link adds a vital 3.5km (2.1 miles) to regional plans to link Asia by rail, ferrying goods and passengers.
The UN-backed Trans-Asian Railway now has nearly 74,700km of working track serving 29 countries, and estimates for completion range from 10 to 15 years.
The inauguration marked the first international rail link for Laos.
A Laos foreign affairs spokesman said the railway was important for his country as it would greatly reduce export costs, as transport by lorry was only previously available.
The ultimate aim was to "unlock" and transform a country with no direct access to the sea, Lao Railway Authority spokesman Sompong Pholsena told the Lao News Agency.
The Thai Lao Friendship Bridge now has a functioning rail link
The track over the Friendship Bridge took 20 months to complete and was funded by Thailand at a cost of about 197m baht ($5.5m; £3.8m).
The bridge is a key part of the region's transport development, extending road links from Singapore to China's port city of Shanghai.
The new network is part of a plan conceived in the 1960s to connect Asia with a continuous railway, stretching west to Turkey and Russia, and north and east to China, Vietnam and South Korea.
Decades of regional conflict and poverty have delayed the plan until recent years.