That's exactly the same mistake Toronto has made, and continues to make, with the badly needed DRL.
.
Modest service, at reasonable cost, reasonably soon, is far more useful to the public than a gold-plated pie-in-the-sky dream in never-never land.  So, now, on the DRL its 30 years waiting, and another 25 years of waiting yet to come.
.
In the meantime, 40,000 riders/hour go 19 stops to Finch with their faces pressed against the doors - all because the TTC is too pig-headed to rebuild the CP Don Branch, which would give at least interim relief for most of the YUS problem - for pennies on the dollar compared to the cost of the DRL.
.
Build ordinary electrified rail service, at 120 - 160 km/h, and make it run reliably on 20 or 30 minute intervals, as soon as possible.  Then, over time, steadily and incrementally upgrade the service.
.
Otherwise, the job becomes a political football, and nothing ever gets done.
.
DRL: 30 years waiting.
Only 25  more years to wait.
.
If we're lucky...