Line 4 ends with a semicolon. Semicolons can be used to join two independent clauses. In this case, lines 1-4 are joined to lines 5-8. The first four lines describe the changes of the landscape; the buffaloes used to roam where the trains now sing. Lines 5-8 add to the description of that landscape.
Line 12 ends with a colon. Colons are usually followed by a list or example that is related to the clause before it. Lindsay repeats that the buffalo are gone with the amendment that some remnants of the past remain. The buffalo are gone but the Blackfeet and Pawnees (like the prairie flowers) “lie low.”
Lindsay uses the semicolon to join two similar but independent clauses. He uses a colon rather than a semicolon in line 12 because he’s not just comparing two clauses. He wants to emphasize that some things are gone but some things still remain. This poem is about nostalgia but also hope in the future. The train “sings” and the buffalo “bellow.” This connotes nostalgia for the past but an optimistic look at the future. His point stands out more with a colon than with a semicolon or a dash. The buffalo are gone and the landscape has changed: but the Blackfeet and Pawnees still lie low.