The three countries are prepared to ink another agreement this week at Beijing's Belt and Road summit.

Formal talks on the project - a rail line expected to stretch 873 km (542 miles) - began in 2014 but have been beset by delays, including disagreements over design, financing AND technical assistance.

The Thai project is part of China's plan for a network of links across Southeast Asia that would eventually connect Kunming in southwest China with Singapore.

It is also part of a broader Chinese initiative to build infrastructure to connect China with Asia, Europe and beyond.

Thailand decided in 2016 against Chinese financing for the project because of high interest rates - a complaint of similar projects in several countries - and decided to fund the 170 billion baht ($5.32 billion) Thai portion of project itself.

So far only the first 3.5 km of the line have been constructed in Thailand, but a Transport Ministry official told Reuters on Tuesday the first section leading to Bangkok should be completed in two to three years.

The project will be re-energised when Thailand, Laos and China sign a three-way memorandum of cooperation on Thursday at a Beijing conference to build a railway bridge connecting the Thai province of Nong Khai and the Lao capital Vientianne, said the Thai Ministry of Foreign Affairs.

"The connecting bridge will make the project an example of seamless connectivity in the region," Lada Phumas, director of the ministry's East Asia division, told reporters at a news briefing.

"The project is going at its own pace. We must stress that the project is making progress according to our goals," she added.

The Belt and Road summit, which takes place from Thursday to Saturday in Beijing, will be attended by Prime Minister Prayuth Chan-ocha and his foreign and transport ministers.

The comments from Thailand come after Malaysia and China agreed on April 12 to resume construction of a 688-km (430 mile) rail project.

'Really happening'

The Thai-Chinese railway is divided into two sections: the first is a 250-km (155 mile) line linking the Thai capital Bangkok and the northeastern province of Nakhon Ratchasima.

That section is expected to be operational in two to three years, Chaiwat Thongkamkoon, permanent-secretary of the Ministry of Transport, told Reuters this week. He could not provide a timeline for the completion of the full project.

The other part links Nakhon Ratchasima and the Thai border at Nongkhai province, where the bridge - the subject of Thursday's agreement - will connect the Thai rail with the Laos network.

The construction of parts of the high-speed rail is done separately in each country. Thailand says it is not accepting financing from China, but using Chinese expertise and buys equipments and rail technology from China.

"That is why this connection area (the bridge between Laos and Thailand) is important. It symbolises that the Belt and Road initiative through this southern corridor is really happening," Chaiwat said.

He said back-and-forth discussions between China and Thailand have caused delays, but negotiations on the highly technical train system - consisting of signalling, power, and track-work - are now near completion.

"The Chinese developed the rail initially for their domestic use, and they have come far to export such technology but it is still relatively new for them in transferring their technology to others," Chaiwat said, adding that most of the documents, training courses and design were initially in Chinese.

"The negotiation is 90 percent completed and I think a deal can be tabled and signed by both sides in a month's time," Chaiwat said. "After that it's all about construction."