Tongking

Then he went to lndo-China, and travelled to Tongking, Annam, Cochin-China, Cambodia, and Siam. He journeyed overland from Hanoi, in Tongking, to Northern Annam; and while in Siam he spent some days at Angkor-Wat, on the shores of the Great Lake, where he examined the ruins of the vast and stately temple with the meticulous care he had taken at Persepolis.

Source: wiktionary

The most important was the Indo-China route, as it was the only one which possessed a railway—the French-owned Indo-China Yunnan Railway, a metre-gauge, single-line track running from Hanoi, the capital of Tongking, to Kunming, as well as a second line from Hanoi to Nanning.

Source: wiktionary