It should first be said that this passage does not refer to the Hudson River which, as it located in the eastern United States, is not prominent within the narrative. The "Hudson" in this passage refers to the Joad family's converted Hudson Sedan, which is by and large the only functional possession that is left unto them and is their "hearth" as they journey west towards California.
This dense and sprawling selection of prose is just one example of Steinbeck's descriptive genius that permeates The Grapes of Wrath. Notice how article adjectives are completely eschewed from the description. This subtle tweak of language portrays the sedan in almost religious terms. "With bent and scarred radiator" sounds almost biblical compared to the casual "with a bent and scarred radiator." Language such as this brings to mind works such as T. S. Eliot's "The Wasteland," which, written nearly two decades prior, described London in almost apocalyptic terms. Steinbeck is frequently credited with showing America in a similar light.
You could easily be forgiven for thinking that the Hudson refers to a river, so vividly is it described in archaic and mystical language. The term "ancient" reverberates with the reader. Where "old" would have simply served to describe the physical condition of the car, "ancient" implies something arcane, a ward against the darkness of financial and physical ruin that surrounds the family, an heirloom that guides them to freedom.
The rest of the passage describes the worn nature of the vehicle that is equal parts disdainful and reverent. The criticism for the shoddy condition of the vehicle is there; however, the age represents a power in its continuing function. The car seems to be wise, filled with knowledge of years past. "Caps of red dust" is an almost hyperbolic descriptor that portrays the car as some ancient landscape, still standing despite years of use and abuse.
All of these descriptors come together to form a cohesive idea of the car. It is indeed a hearth, one almost more sacred and central than a literal hearth could ever be. It is old, bent, and in terrible shape, but it is a lifeline of incredible value and almost spiritual significance.