An All-Red Route was, originally, a steamship route used by Royal Mail Ships during the heyday of the British Empire. The name derives from the common practice of colouring the territory of the British Empire red or pink on political maps.
Initially the term was used to apply only to steamship routes (as these were the only practical way of carrying communications between Great Britain and the rest of the Empire), particularly to India via the Suez Canal--a route sometimes referred to as the British Imperial Lifeline. Rail transport was used across France and Italy to the Mediterranean. From 1868 to 1871 the Mont Cenis Pass Railway, a temporary mountain railway line over the Mont Cenis Pass was used for mail.
After use of steamships became widespread at sea, strategically placed coaling stations were acquired to guarantee the mobility of both civil and naval fleets.
In the 1880s the term "All-Red Route" was expanded to include the telegraph network (see All Red Line) that connected various parts of the Empire, and by the 1920s it was also being used in reference to proposed air routes, initially airship and then flying boat, between Great Britain and the rest of the Empire, see Imperial Airship Scheme.
The Suez Canal route dramatically shortened the sea path between Britain and its colonies in Asia, and, conscious of its significance, the British sent troops to seize control of the canal in 1882 at the beginning of the British occupation of Egypt; even after British troops were withdrawn from the rest of Egypt in accordance with the Anglo-Egyptian Treaty of 1936, Britain continued to control the canal and kept troops stationed in the canal zone. After Egyptian President Gamal Abdel Nasser nationalised the canal in 1956, sparking the Suez Crisis, UK Prime Minister Anthony Eden declared that "The Egyptian has his thumb on our windpipe", describing the Suez as the "great imperial lifeline".
The major "All-Red Route" ran as follows:
Southern Britain -> Gibraltar -> Malta -> Alexandria -> Port Said (after construction of the Canal) -> Suez -> Aden -> Muscat (and access to the Persian Gulf) -> India -> Sri Lanka -> Burma -> Malaya -> Singapore (branching out into the Pacific Ocean towards Hong Kong, Australia, New Zealand, and other British colonies).
With the end of the British Empire and the increasing prevalence of air travel, the terms "All-Red Route" and "British Imperial Lifeline" have fallen from use, and now exist largely in a historical context, generally in reference to the routes in use during the British Empire.