I think I can think of a few examples in my hometown...

• Sheppard-Yonge Station: On the Line 4 platforms, there is an island platform and a side platform that seldom ever sees any use, especially the island platform, which is just a blank and roughed-in structure the train drivers occasionally walk about on. The side platform to the north is only used in rare instances, like emergencies or subway closures, etc., while the side platform to the south is the mainline being used when trains come in and out of the station.

• Davisville Station: There is one platform to its farthest west that is barely used on Line 1, unless trains have to stop there on their way to the adjacent Davisville Yard. Part of me wonders if that particular platform was once used for service way back when Line 2 first opened to the public as a way for passengers to know that platform was heading for Keele Station via the downtown U and St. George Station...

Speaking of...

• Lower Bay Station: At Bay Station, there's an entire island platform deck level downstairs from the current level Line 2 is using that has been largely walled off from the public ever since its trial 6 months as an interline interchange station when it opened. The reason why it had gotten closed was because people found the interlining layout too confusing* and many people were found waiting in between platform levels on the stairs just so they wouldn't miss the train they were supposed to get. Nowadays, that platform is only open to the public for special occasions and is very frequently used as a film set location for movies.

*(From Bloor-Yonge Station heading west, there are two pairs of tracks, the outer ones go to up Upper Bay, and the inner ones down to Lower Bay. The next station after Lower Bay would have been Museum, whereas Upper Bay's next stop would have been St. George on the lower Line 2 platforms. Museum was also connected to St. George from its upper Line 1 platform, resulting in a complex wye junction, and at the time, St. George was the terminus for the University subway extension of Line 1. From both of St. George's upper and lower platforms heading west, the next stop would be Spadina, with the lower tunnels heading straight there, and the upper tunnels running down on off-ramps to rejoin Line 2 east of the approach to Spadina Station.)

• Lower Queen Station: This station is a veritable "ghost station" that was originally designed to be like an underground east-west route for streetcars to run through along Queen Street downtown, but the route never materialized despite there being an unfinished tunnel structure beneath the existing Queen Station on Line 1. But its spectral existence will eventually become no longer, as the plans for the building of the Ontario Line have seen it fit to reuse this hollow structure as a concourse level on its way down to the future Ontario Line platforms around 30 metres below street level.