Physical meaning of the phrase "Beijing-Tianjing corridor" in the context of Sino-Japanese war?

Question

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.

Answer 1

In general, "corridor" denotes a linear geographic region with one or more major modes of transportation running its length. Usually its existence points to two very important locations at its ends, and it promotes development along its length due to the heavy traffic. In Beijing-Tianjin's case, there was a railway between the two cities (to which the Marco Polo bridge was very close).

China's few railways took on outsize importance during the early Sino-Japanese war, due to the low-to-nonexistent level of other infantry motorisation. Control of railways was the key to rapid manoeuvre and linking up distant forces in that theatre. In the case of the Beijing-Tianjin railway, the fall of Tianjin meant that Japanese forces there, composed both of the sizeable garrison prior to the war and those just landing by sea, could rapidly transport to Beiping (Beijing), which the Japanese took shortly after.

Answer 2

The significance of the Beijing-Tianjin corridor is, as you said, "Beijing's main maritime gateway."

It first obtained this significance in the Arrow War (Second Opium War) of 1856-60. After several years of desultory fighting, the British struck a decisive blow by capturing the coastal forts of Tianjin, which allowed them to land enough troops for a push to Beijing, forcing an end to the war.

A second time this became important was during the Boxer Rebellion of 1901, when the "eight" (foreign) nations used Tianjin as a "staging" point for a relief expedition to foreign legations besieged by the Boxers in Beijing.

As late as 1927, there were U.S. troops on "standby" at Tianjin during the Northern Expedition" of Chiang Kai Shek, ready to protect western interests in Beijing if necessary. Chiang, whose wife was American educated, didn't make this "necessary."

The significance of the occupation of the whole corridor by Japan during the Sino-Japanese war meant that e.g. America could not use Tianjin as the starting point of a "policing" action against Japanese forces in Beijing, but rather a major campaign would be necessary to recapture this city, probably preceded by the (difficult) recapture of Tianjin itself.

The "physical" meaning of the corridor has changed over time. In the time of the Arrow War, it just referred to "roadways" connecting the two cities. By 1937, it contained one of China's few railroads. Nowadays, there is a high speed train running between the two cities.