A starting point for answering Geoffrey's question is that many of the stations in the Perth metro area and on the line to Bunbury were not very substantial before they were demolished or modified.
On the Perth to Midland line, several stations were modified significantly at the time the line was changed to dual gauge (late 1960s), and the old Midland station was demolished and relocated. Also, the East Perth loco shed was demolished and replaced by East Perth Terminal.
On the Perth to Fremantle line, Fremantle station was similarly (partly) demolished in the late 1960s because of the standard gauge project. Some time later, North Fremantle was relocated and Leighton closed, and West Perth was closed, demolished and replaced by City West.
On the Armadale line, several stations, eg Kelmscott, have been redeveloped (Kelmscott became a train-bus transfer point).
Other stations in the Perth metro area had wooden bus shelter type buildings replaced with brick ones (eg West Leederville, Mosman Park).
In the 1980s, extensive changes were made to Perth station, but the old station building was retained and is still there.
Further modifications were made in the metro area at the time of electrification at the start of the 1990s, eg Claremont became an island platform station (although the buildings were left as they were). At about the same time, changes to signalling equipment led to the relocation of some signal boxes to other places (eg Whiteman Park) and the demolition of others (the affected signal boxes included Fremantle, Subiaco, Perth and Claisebrook).
More recently, Subiaco station was relocated underground.
Some Perth metro station buildings and platforms have not been modified much since the end of steam operations (eg Bassendean, Guildford and Daglish).
On the Armadale to Bunbury line, Bunbury station was relocated in the 1980s, and other station buildings (eg Brunswick Junction) were demolished at the time the yards adjacent to them were removed.
There are still some quite substantial old station buildings out in the wheatbelt on other lines, eg on the Avon Yard to Albany line at York, Beverley, Pingelly, Mt Barker and Albany.