The last two platforms added were 5 and 6. The photo I referred to earlier clearly shows platforms 1 to 4, with 5 and 6 absent. Interestingly, the pedestrian subway at the up end had already been built to accomodate the two extra platforms - which means that the stairs leading to platform 6 predated the platform.
I'd be interested to know how Burwood was built and changed over the years. I have my speculations, but it'd be nice to hear from someone who knows.
My speculation is that the original line ran roughly where the local lines are now, except at ground level. On the down side of the Burwood Rd level crossing was the original station. The old goods office is still there, and I've been told that this building was the original station, but I don't know if that's accurate.
With quadruplication, the main and suburban Burwood Rd bridges were built, along with existing platforms 1 to 4. All original platform buildings from this period have since disappeared, except for parts of the entrances at the up and down ends. The original lines still ran during construction, probably through or adjacent to the old goods yard. (In other words the four new tracks formed a sort of deviation from Conder St to Shaftesbury Rd).
Once the four new tracks opened, the old ones closed, with the part on the down side of the goods platform becoming part of the goods yard, and the tracks on the up side of the goods platform lifted.
With sextuplication, the old lines were built over to form the current local lines and platforms 5 and 6, and the local lines bridge was built. I don't know whether the buildings currently on the northern side of the corner of Railway Pde and Burwood pre or post dated sextuplification.
I stress that all this is complete speculation (other than my first paragraph). Anyone have the facts?