I think it’s correct as-is. Inserting a “were” would make that clause read as independent. With how the sentence is currently structured, that doesn’t work.

That’s not to say you couldn’t have

The tracks are now unruley [sic] and wild—the people once tied to them were killed in crosswalks by giant trucks

if you want, but the comma needs to change to something like a dash or a semicolon. With a comma (i.e., as a subordinate clause), “were” doesn’t make sense.