Describing the beginning of the Sino-Japanese war, especially the events following skirmishes after the incident at the Marco Polo bridge in 1937, my textbook stresses
Soon the Beijing-Tianjing corridor had fallen to the Japanese.
This phrase is found repeated in the summaries of two different professors, hence I suspect it must be of great strategic significance. I examined three different maps of China for any feature of geostrategic importance and found none. Hence, I suspect it refers to a railway or other kind of supply line. The significance of such supply line would be clear, as Tianjin is Beijing's main maritime gateway.
So, could someone explain what kind of "corridor" was present in 1937 between Beijing and Tianjin, that the Japanese took?
Or was it simply that they conquered everything north of and including Shandong Peninsula, thus isolating Beijing from the sea?
EDIT: Although I sincerely appreciate the elaborations on the significance of the Beijing-Tianjin connection, my original inquiry was, what kind of link there was present, which is called "corridor". In other words, did the Japanese capture a single railway, a whole area, fortifications rendering the railway useless... Presumably, due to my inferior language skills, the question was edited to suggest a different meaning.