Literarily, this may refer to the opening paragraphs of Joseph Conrad’s ‘Heart of Darkness’:
The sea-reach of the Thames stretched before us like the beginning of an interminable waterway. (…) [I]n the luminous space the tanned sails of the barges drifting up with the tide seemed to stand still in red clusters of canvas sharply peaked (…).
The setting of Conrad’s novel is a sailing boat (a ‘yawl’) approaching the threshold where the river Thames enters the English Channel.
Literally, the red sails could easily be associated in Eliot’s day with the Thames sailing barge.
These small trading boats were very common at the turn of the 19th and 20th centuries, until after WWII. Their sails had a rusty-red colour, due to the significant amount of red ochre used to make them water-resistant. Their small size allowed them to float in the narrow creeks of the Thames and navigate under London bridges, to trade all sorts of materials — mainly bricks, mud, hay, rubbish, sand, coal and grain — needed at the time when the city was rapidly expanding.
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