Punctuation is a stylistic aid to guide the reader to the correct parse of linear text. In the hands of a master, it can also guide a reader to the vision that the author intends to convey. Let's start out by listing what Ellison's narrator sees:
- empty fields
- red clay roads
- a river
- yellow algae
- more empty fields
- a railroad crossing
- disabled veterans
- a legless man in a wheelchair
In the hands of an untalented author, this list of viewed objects could simply be spilled out, each separated from the next by a comma. But that's not what Ellison does. First he gathers three initial sights:
- beyond the river to the shacks
- past more empty fields
(Notice that 1 is a direct object, while 2 and 3 are prepositional phrases, which give a sweep to the narrator's vision.) Secondly, he places the other images mostly as adjuncts or parts of adjuncts (i.e., modifiers) to the three:
- cabins surrounded by empty fields
- beyond the river, sluggish ... covered by algae
- past more empty fields to the ... shacks at the railroad crossing where the ... veterans visited ..., hobbling down the tracks
This is still a list, but the second has an internal comma (between river and sluggish), so it would be misleading for a comma to separate 1. beyond the river, and 2. past more empty fields: a reader might expect another phrase or clause modifying river. So a semicolon stands in stead to separate the second sight from the third.
Now we come to the last element, the gerund clause
sometimes pushing the legless, thigh-less one in a wheelchair.
Grammatically, this is a nominative absolute, a participial clause loosely connected syntactically to its associated clause
veterans visited the whores
describing both the subject (the veterans) and the predicate (their visiting) by giving the manner of their action. Ordinarily a comma separates an absolute from its associated clause, but Ellison further loosens the attachment with a semicolon, stylistically making it part of the preceding list and suggesting that it's the final, fourth sight in the narrator's vista.