I can think of a couple locations:
La Trochita (the Old Patagonian Express) at Ingeniero Jacobacci station is a dual gauge. (Broad gauge (5'6") and 750mm) There are also
16km of dual gauge mainline between Ingeniero Jacobacci and Empalme where the 2 lines split.
Dual gauge at Port Elizabeth, Port Shepstone and Donnybrook yards and on some mainlines of South African Railways (3'6" and 2' gauge lines).
Several miles of dual gauge track between New Jalpaiguri and Sukna on the Darjeeling Himalayan Railway (5'3" gauge and 2 foot gauge)
More dual gauge in Bosnia, former East Germany around Dresden (and other cities), China, India, Austria, South America ...
Best Regards,
Glenn