How about a railway cutting dug in 1847, but never saw any rails?
Similarly there's various remnants of the never completed Ouse Valley Railway (LBSCR) from the Brighton Main Line to Uckfield (and thence beyond to Polgate). Construction (including bridges (or bits of them!) and embankments still extant) started May 1866; abandoned Feb 1867.
I can think of a few more examples of infrastructure that was never used (which I don't think have been mentioned up-thread) :
Shalford Junction, south of Guildford, embankment for an east to south curve, built as part of territorial skirmishes around the Portsmouth Direct, no track ever laid.
London Underground Northern Heights extension, delayed by WW2 and finally abandoned a decade later - station works at Finsbury Park and Highgate high level, and a part built viaduct in a field north of Edgware. The intended depot at Aldenham was used for aircraft building then bus overhauls, but has since been demolished.
South of Beddgelert in Snowdonia, a bridge over the A498, and two abutments in the middle of a field, built by one of the forerunners of the Welsh Highland, which was later built on a different alignment.