The money spent was more than what the voters approved. Making a new estimate that is much higher and spending less than that isn't coming in "under budget".
The initial light rail was given an estimate of 1.8B of that 3.9B that voters approved, and year of expenditure costs were roughly 20% higher then that, but federal studies by the dept of transportation have evaluated the costs and believe that there was a period of unpredictable and unusual inflation. If you control for that, the project would have completed under it's original budget. Additionally, they did come under their revised budget and the Ulink extension has come in under budget too.
I guess I don't fault a transit authority for making a 10+ plan that makes reasonable assumptions about inflation given the information they have, and then deliver only slightly above budget after being impacted by said inflation.
The Ulink extension was part of the original proposal, so it should be counted against the original budget and timeline as well. The U-district station didn’t open until 2016, 20 years after being voted on and 10 years behind schedule. The extension also cost $1.8 billion on its own.
From the same wiki page I cited earlier:
> The ballot measure failed to pass on March 14, 1995, and the light rail line was shortened to 25 miles (40 km), between the University District and Sea-Tac Airport. Voters approved the $3.9 billion package on November 5, 1996
> The initial segment identified and approved by Sound Transit later that year shortened the line to 14 miles (23 km), between Downtown Seattle and a southern Tukwila station near Sea-Tac Airport. The remaining routes to the airport and University District were sent back to the planning stage, and re-organized into separate light rail projects.
> Planning of the Portage Bay tunnel between Capitol Hill and the University District was suspended due to higher than expected contractor bids, attributed to difficult soil conditions.
> The line was extended north to University of Washington station, via Capitol Hill station, on March 19, 2016, via a $1.8 billion, 3.15-mile (5.07 km) tunnel.