May 18, 2018 8:00 a.m. ET By Trefor Moss, WSJ PANG-ASOK, Thailand— Li Guanghe has built some of the most technically complex railroads in China. Now he faces his toughest challenge yet: working abroad to deliver a flagship rail project in Thailand that could make or break China’s hopes of selling high-speed trains abroad. Exporting high-speed rail is one of the ambitious elements of President Xi Jinping’s Belt and Road initiative, which aims to upgrade trade and transport networks from Africa to the Pacific. But despite throwing its diplomatic weight behind high-speed rail, China has struggled to convince would-be partners to commit to the costly technology. Singapore Express China is planning to link Kunming to Singapore by high-speed rail, with a route that passes through Pang-Asok. After years of stalling, Thailand in November started work on the $5.5 billion first phase of a high-speed railway that Beijing hopes will eventually form part of a China-to-Singapore route—a potentially lucrative prize for Chinese rail contractors. Now the heat is on Mr. Li to complete the initial 157-mile stretch connecting Bangkok with the city of Nakhon Ratchasima. “There’s a lot of pressure,” said Mr. Li, a chief engineer with state-run China Railway DesignCorp. , and the man in charge of the project. Construction vehicles rumbled over the soil of this freshly turned site in Pang-Asok, a small rural town in northeast Thailand surrounded by cornfields and foothills. “The China-Thailand railway is an important part of making the Belt and Road initiative successful,” Mr. Li said. “We want this project to be the best it can be.” China’s domestic high-speed network is the world’s biggest, spanning over 15,000 miles. The World Bank credits the massive public-works effort with boosting China’s socio-economic development. Construction at home is set to wind down over the coming decade, however, sharpening China’s interest in exporting railways—preferably sooner rather than later. Chinese diplomats are “making the sales pitch everywhere,” said Jonathan Hillman, a fellow at the Center for Strategic and International Studies, a U.S. think tank. In doing so, they don’t always pause to consider whether the countries they’re pitching high-speed rail to really need it, he said. Thailand and Laos are the only places where China is making concrete progress, and in these countries Chinese officials needed a decade of negotiations to extract firm commitments from their reluctant counterparts. Li Guanghe is the man in charge of the rail project in Thailand that could make or break China’s hopes of selling its high-speed trains abroad. Photo: Trefor Moss/The Wall Street Journal The proposed 2,200-mile Pan-Asia Railway would run south from China to Singapore, via Laos, Thailand—its 1,000-mile central section—and Malaysia. The system would be completely new; there is no high-speed rail in Southeast Asia today. The network was originally conceived by the Association of Southeast Asian Nations, but “China is the one pushing for it to happen now,” according to Agatha Kratz, an adviser at the Rhodium Group consultancy who has written on China’s so-called railway diplomacy. “It’s made the plan its own.” The project has been plagued by doubts over passenger demand and lack of popular support. “I won’t be able to afford a ticket on the high-speed trains,” said Sompot Kaewlaharn, a 53-year-old laborer in Pang-Asok. The old trains are slow, he admits, but like many rail services in Thailand, they’re also free to use. “It seems the government’s doing this because they want to curry favor with China, because China’s powerful,” he said. That view is echoed by a Thai official with knowledge of the negotiations, who said China leaned heavily on the Thais to ensure the railroad went ahead. A model of a Chinese high-speed bullet train sits on display in Pak Chong, Thailand, Dec. 21. Photo: Li Mangmang/Xinhua/ZUMA Press The Thai government under Prime Minister Prayuth Chan-ocha first approved the China-backed high-speed plan in 2014, the year the former general overthrew Thailand’s elected government. The move came amid ambitious promises by the new leadership to overhaul the country’s aging infrastructure. Then came the collapse of talks about how much interest Thailand would have to pay on Chinese loans to fund the railway. In 2016, Mr. Prayuth disappointed Beijing by announcing that Thailand would self-fund the first phase and use Thai contractors, relying only on China for equipment and technical assistance. “The Thais did everything they could to delay,” said Ms. Kratz. The Chinese pushed back. At the G-20 Summit in Hangzhou in late 2016, which Mr. Prayuth was attending at the hosts’ invitation, he was harangued by successive Chinese officials unhappy with delays in getting the project launched, according to the Thai official, who was present. Over the course of the summit the Chinese diplomats wore Mr. Prayuth down until he agreed to push the start button, the person said. China’s Ministry of Foreign Affairs and a spokesman for Mr. Prayuth didn’t respond to questions. A construction crew does ground-preparation work for the high-speed railway line, in Thailand’s Pang-Asok earlier this year, which will run parallel to an existing track. Photo: Trefor Moss/The Wall Street Journal Mr. Li landed in July with a 58-month contract and orders from Beijing to get the railway built. He leads a team of 50 Chinese specialists and 400 Thai contractors, some of whom were working on a hot February afternoon to flatten the first two-mile strip of earth. Mr. Li once built a notoriously difficult railroad between the Chinese cities of Yichang and Wenzhou across boggy, mountainous terrain. The Thai project is relatively straightforward, he said, but here the difficulties are cultural and political, rather than technical. Operating as a foreign guest makes for agonizing progress, he said. Mr. Li’s senior partner in the project, Veerayuth Kaewsawang, a project engineer for the State Railway of Thailand , agreed that it can be painstaking work as they discuss every little twist and turn in the route through translators. “We negotiate about almost everything,” he said. While China has piled pressure on its neighbors to support its high-speed rail ambitions, its desperation to land orders has given smaller countries leverage.