There are pics of them here http://railgallery.wongm.com/protective-services-officers/Yep, they're the ones!
This site http://ptv.vic.gov.au/getting-around/travelling-safely/train-safety/victoria-police-protective-services-officers/
has a list of the 70 stations where PSOs are currently operating.
They're too lazy to put up police ribbons on structures, and you'd inevitably get community complaints that they uglify the train station.You mean building new boxes uglify the stations, right? Using existing buildings doesn't uglify anything...
That, and it'd bring attention to the fact the station buildings are unused.