What is the significance of the discussion of the building of the railway in Heart of Darkness by Joseph Conrad?

When Marlow travels to the Congo, he takes a boat up the big river and, after a few miles, arrives at a place where men are building the railway. The horrible sight of the workers’ poor health and captive condition is offered as a stark contrast to the privileges and power of the white overseers and bureaucrats. The scene also foreshadows his later encounters with Kurtz and the people at his camp.
When Marlow hears explosions before he sees anyone involved in the railroad project, he concludes that workers are blasting through the rock but that the activity is basically busywork, as the cliff is not actually in the way. Marlow vividly describes the men who are employed on the railroad project and assesses the value of the project itself and their involvement in it.
In these passages, Marlow condemns the excesses of colonial rule and, more generally, the entire colonialist enterprise. He particularly criticizes the dehumanization of black workers, beginning with the six men he sees on a chain gang. They are not chained at the ankles or even around the waist, but by their necks. This is apparently the first time Marlow has personally witnessed such brutal treatment by the colonial rulers of the colonized people. He understands that the foreign authority is imposed upon them but not understood; he calls it a "mystery from the sea." Marlow watches six bone-thin African men walking together, carrying baskets of dirt on their heads, and hears the chains clink.

I could see every rib, the joints of their limbs were like knots in a rope; each had an iron collar on his neck, and all were connected together with a chain whose bights swung between them, rhythmically clinking. . . . [T]hese men could by no stretch of imagination be called enemies. They were called criminals, and the outraged law, like the bursting shells, had come to them, an insoluble mystery from the sea.

A single white man with a rifle is overseeing them, and when he sees Marlow, he quickly decides he is no threat and approaches him with a grin. Marlow understands that his whiteness creates, in this man’s mind, a bond between them. The ironic tone of the last sentence below indicates Marlow’s negative assessment of the men’s captivity and forced labor, which he refers to as “high and just proceedings.”

. . . with a large, white, rascally grin, and a glance at his charge, [he] seemed to take me into partnership in his exalted trust. After all, I also was a part of the great cause of these high and just proceedings.

Marlow is deeply disturbed by the sight of the chain gang. He compares the impression it makes to other horrible sights he has seen before, calling those things “devils.” He anticipates, however, that the devil he will encounter upriver will be of a different kind, based on greed and having no pity. This incident serves as foreshadowing for his meeting with Kurtz later in the novel.

I've seen the devil of violence, and the devil of greed, and the devil of hot desire; but, by all the stars! these were strong, lusty, red-eyed devils, that swayed and drove men—men, I tell you. But as I stood on this hillside, I foresaw that in the blinding sunshine of that land I would become acquainted with a flabby, pretending, weak-eyed devil of a rapacious and pitiless folly.

As Marlow moves uphill and ducks into a shady spot, he quickly realizes that he has not seen the worst by any means. Here he sees men who are sick and dying; they are so weak they can never escape and so are not chained but “free.” Marlow’s description of his own attitude of “horror” also foreshadows Kurtz’s dying words near the novel’s end.

They were not enemies, they were not criminals, they were nothing earthly now—nothing but black shadows of disease and starvation . . . These moribund shapes were free as air—and nearly as thin . . . all about others were scattered in every pose of contorted collapse, as in some picture of a massacre or a pestilence. . . . I stood horror-struck . . .

The contrast in his next meeting is even more vivid than with the unnamed overseer. A very well-dressed white man rushes up to him. He is the Company’s chief accountant. Marlow ironically comments on the “achievement” of “keeping up appearances” in such conditions.

I respected the fellow. Yes; I respected his collars, his vast cuffs, his brushed hair. His appearance was certainly that of a hairdresser's dummy; but in the great demoralization of the land he kept up his appearance. That's backbone. His starched collars and got-up shirt-fronts were achievements of character.
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