My first guess was going to be Canterbury Road Canterbury (former level crossing and now rail overbridge) to the point where the outer circle went under Canterbury Road but this is approximately 1km.
I'll guess, without any certainty, Dynon Road (Goods lines over / Dock Link Rd) to Maribyrnong River (old Maribyrnong River line under) and assume there may've been a level crossing there somewhere previously?
Good question