"Were being built" describes a situation in the past when construction was on going over a period of time. We are talking in general here, not about a specific railway or road or canal. All of them were coming together, rapidly.
"Were built" describes, or implies, each road, railway and canal was, itself specifically, built rapidly. We can imagine a loss of quality, for example, in the rush to complete each project, an implication that is missing when you say they "were being built at a rapid pace."
If I were to animate this into a Disney cartoon, the first example would show a map of the country, with roads, railroads, and canals being drawn on the map in an increasingly dense network of lines. Eventually, the map would be more lines than open spaces.
The second example, however, would open as a cartoon of an assembly line in a factory, churning out trains as quickly as Donald Duck and his compaions could assemble them; then switching to an outdoor scene as Mickey and his pals were rapidly digging canals, pitching shovelfuls of earth over their shoulders just as the water was coming down the canal carrying a boat. Meanwhile another set of workers would be grading road surfaces right up to the canal bank.
Does the visual help?
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