While this 401 tunnel idea is somewhat crazy (I for one would rather see more GO branches to improve coverage, HSR, adding truck lanes, and/or a 160km/h express orbital metro line along the 407) I would like to write a bit more on Sydney’s tunnel construction for context, as it is much more than the massive WestConnex tunnel which other posters have brought up.
In the last ~30 years, Sydney has constructed:
1992: Harbour Tunnel, 2.8km
1999: Eastern Distributor, 1.7km
2001: M5, 4km
2005: Cross City Tunnel, 2.2km
2007: Lane Cove Tunnel, 3.6km
2020: NorthConnex, 9km
2019-2023: WestConnex, 26km
And there’s more u/c:
2025 M6 Tunnel, 4km
2028 Western Harbour Tunnel, 6.5km
(Dates are opening dates, lengths are for tunneled sections only)
That’s ~60km, with even more in planning! Other than the harbour tunnels, these are all really urban tunnels underneath suburban and urban areas and not any geographic barriers.
There’s no shortage of complex engineering works either - underwater crossings, giant underground interchanges, deep tunnels under other tunnels, etc.
One extremely big thing to note however is the very significant private money put in projects plus private operation of those tunnels with tolls. You do see familiar names frequently in these consortiums - CPPIB, CDPQ. I could honestly see some kind of deal working out for significant financing from some combination of the 407 consortium, CPPIB, CDPQ, and Bay St/Wall St to partially fund a toll 401 tunnel similar to the Sydney projects - such a tunnel would no doubt be a huge money printer once constructed, if tolls are not too restricted. The 9km NorthConnex, for instance, had only 1B AUD of government cash funding, with the remaining 2B privately funded or funded in the form of extended toll concessions for other private toll roads. That said, Ford did ban new tolls on provincial highways, so I have no clue if this is on the table. But if the government can get a decent deal with significant private funding to foot some/most of the bill, I don’t think it would be the worst idea.