Toronto, Canada, has 2 unused underground stations. The article mentions Lower Bay which is a relic of the original interlining when the Bloor Danforth line was built. (1966) There was a triangular operation using the two new termini and one of the original line's. The junction wye used a pair of 2-level stations on the BD line. The lines were operated as integrated for 6 months, then separately for 6 months, which has now become 48 years. St George station became the terminus for the Yonge line trains and an interchange with the BD line. Lower Bay was blocked off but used for stock transfer and occasional film work. The Yonge line was later extended through St George and northward. The wye is in the running for "most expensive piece of track per passenger served".
There is also a roughed-in but never used station under Queen (at Yonge). This was built in the 40s/early 50s with the Yonge subway when it was thought that the Queen streetcar would be put underground. It's still proposed sometimes. The platforms are used as an underpass between north and southbound platforms, and for storage. I've seen the tunnel when the access doors have been open.