South Korea has said the Thai government is backing away from a promise to grant it rail project investment rights, instead signing a deal with China to invest in the infrastructure.
A South Korean minister queried why Thailand granted the projects to China even though Prime Minister Prayut Chan-o-cha earlier persuaded Seoul to invest in a dual-track railway when he visited late last year.
Prime Minister Prayut Chan-o-cha and his wife try a high-speed train ride during their visit to China in December 2014.
Deputy Prime Minister Pridiyathorn Devakula said he explained to South Korea's Land, Infrastructure and Transport Minister Suh Seung-hwan that the China deal was agreed under the previous Thai government, and that the Prayut government is obligated to honour its commitment.
Mr Suh met Transport Minister Prajin Juntong at the ministry on Monday to discuss the matter, before meeting MR Pridiyathorn, who oversees economic affairs, at Government House later.
Mr Suh is asking the Thai government to keep its promise.
Thailand and China last month signed an agreement to construct a 734km Nong Khai-Nakhon, Ratchasima-Kaeng Khoi-Map Ta Phut route, and a 133km Kaeng Khoi-Bangkok route.
China would finance the projects with around 350 billion baht.
MR Pridiyathorn said there are other opportunities for South Korea.
"Thailand still has plans to develop urgent railway projects 1m wide and spanning more than 700km in order to facilitate goods transport," said the deputy prime minister.
"The government will welcome South Korea's participation in the bidding."
ACM Prajin said, however, that South Korea expressed interest to invest in a 1.43m wide dual-track railway, though the government has not indicated which route.
"The Thai Transport Ministry has agreed to provide details of the projects needed by the country to South Korea by this week for its consideration," he said.
After that, the two countries will hold ministerial-level discussions over the issue next month.
"Gen Prayut will be the one who has the final say on which railway projects will be granted to which country," ACM Prajin said.
According to ACM Prajin, the Korean minister said South Korea has great expertise in high-speed railway development, with trains travelling at up to 300kph.
If the two countries agreed on investment, the cooperation would be on a government-to-government basis, the minister said.
A source at the Transport Ministry said after Thailand inked the railway development deal with China, both Japan and South Korea were irked and approached the government for a similar agreement. Previously, Thailand agreed in principle to grant Japan investment rights on the Tak-Mukdahan rail route to link the country's East and West.
Negotiations stalled after the Japanese government was dissolved.
Japan has also expressed interest in investing in two double-track rail routes, the first from Ban Phu Nam Ron in Kanchanaburi to Rayong and the other from Bangkok to Chiang Mai.
The first route will be the gateway for goods moving from the Dawei industrial zone in Myanmar to Rayong, where there is a deep-sea port serving the Pacific Ocean.
However, the government has yet to make a decision on the matter.
A diplomat source said Gen Prayut is planning an official visit to Japan next month. Among core agenda items is infrastructure investment in Thailand.
Apart from China, Japan and South Korea, Germany has also expressed an interest in investing in a proposed 167km dual-track rail route from Nakhon Pathom to Hua Hin in Prachuap Khiri Khan, according to the source at the Transport Ministry.