During WWII China depended on the Burma road to move supplies from Rangoon to southwestern China. It was built during 1937-38, when it became clear that Chinese ports were in dangered by Japanese advances. However its carrying capacity was limited, in fact a fraction of the Hanoi to Nanning railroad which served until 1940 when the Japanese invaded French Indo-China.
But what if instead of a mountain road, a railroad exited in its place? Such a railroad could link Lashio in Burma to Kunming in China, the same route as the OTL Burma Road. It would be capable of far greater carrying capacity and significantly improve Chinese war making ability.
Around the late 19th to early 20th century, there were several attempts by the British to build this railroad. The benefits include spreading British influence to the Yunnan province of China, bolstering Burma-China trade, and linking Burma and French Indo-China by rail as there is a French railroad from Hanoi to Kunming. However the project was deemed uneconomical at the time.
In 1938 the project finally began and was expected to finish in 1943. However the Japanese invasion of Burma in 1942 killed off the planned route.
But what if construction began earlier? Suppose the Chinese, British, and French pooled money in the early Thirties and finished the railroad by the outbreak of the Sino-Japanese War in 1937. Far more supplies would've made its way to China without being destroyed or stolen along the slow, breakdown prone trip by trucks. The Chinese intervention in Burma would've been quicker and in greater strength. Perhaps preventing a successful Japanese invasion?
http://en.wikipedia.org/wiki/Yunnan-Burma_Railway
http://en.wikipedia.org/wiki/Burma_Road
