The plan, in colonial French Indochina, was a line from Tân Ấp in Vietnam to Thakhek on the Mekong in Laos — the missing lateral that would have tied landlocked Laos to a Vietnamese seaport and made the whole Indochinese network cohere. The obstacle was the Annamite Range, and the answer was to go under it: a system of tunnels driven through the mountains beneath the Mụ Giạ Pass. Work began. It did not finish. Budgets and engineering difficulties ground the project down, and in 1950, with the tunnel still incomplete and the First Indochina War closing in, it was abandoned. The pass has never carried a railway. Three-quarters of a century later, that is still true.

One fragment of the scheme did open. From 1933, Tân Ấp served as the terminus of a short line running to Xóm Cục — the only stretch of the Thakhek–Tân Ấp railway ever put into service, a stub reaching toward a border it could not cross. From there the aerial ropeway took over, carrying goods and ore the remaining distance to Naphao. The historian Martin Stuart-Fox records that the cable line did not survive the First Indochina War. What was left afterwards was a branch railway ending in the mountains at a place where the freight used to leave the ground, and nothing at the far end of it.

Nobody is quite certain when the Xóm Cục branch stopped running. What is documented is that North Vietnam brought part of it — perhaps all of it — back into use during the 1960s, for reasons the French would have recognised. The Hồ Chí Minh trail entered Laos just south of Xóm Cục, through the Mụ Giạ Pass, and Mụ Giạ was the principal doorway for everything moving toward the fighting in the south. A rail line that could put supplies within a few kilometres of the trailhead was worth reopening under any circumstances. It was also worth bombing, and the valley around Tân Ấp did not escape the campaign that followed.

The main line through Tân Ấp is the one the French did complete, though it took them from 1899 to 1936 to do it. On 2 October 1936 the full 1,726-kilometre Hanoi–Saigon link — metre gauge, single track — went into operation. It was cut at the Hiền Lương Bridge in 1954 when the Geneva Accords divided the country, and the decades that followed left it in pieces. Restoring it after 1975 meant repairing 1,334 bridges, 27 tunnels, 158 stations and 1,370 sets of points. The line reopened on 31 December 1976 and was promoted, accurately enough, as a symbol of a reunified country. That is why the trains through Tân Ấp are still called the Reunification Express.

Tân Ấp today is a small station in Tuyên Sơn commune, in what is now Quảng Trị province — Quảng Bình was dissolved into its southern neighbour in June 2025 — sitting on the line between La Khê and Đồng Chuối. Express trains pass it. The interesting thing about the place is not what stops here but what is scheduled to. Vietnamese railway planning now lists Tân Ấp on a projected Vũng Áng–Mụ Giạ line, the eastern half of a revived Vientiane–Vũng Áng corridor: a feasibility study funded in 2015, a letter of intent in April 2017, and in March 2026 a concession agreement covering the 147 kilometres from Thakhek up to Mụ Giạ. The route is essentially the one the French drew. It has taken a century, and the cable is long gone, but the idea has proved harder to kill than the tunnel was.