Just guessing but I suspect that “straightening” that curve would have been nearly as expensive as the rest of entire project.
Reroute the track 3000’ on either side of the highway - and put in an angled bridge that is 6 times the length of the existing one.
Although doing the Environmental Impact Statement for disturbing the hundreds of fir trees (out of approximately 1.5 billion in Washington state) would probably eat up most of the budget - leaving nothing for the actual work.
I did a search of what the price tag is for a railway bridge. Still don’t know - but see that the price has now doubled to get light rail across the I-90 floating bridge between Seattle and Bellevue to the east - $710 million.
Once it gets built I need to remind myself to give them a few months of practice before taking that bridge! The floating bridge is in different sections and floats on the large lake, with the sections able to move a bit independently. I’m guessing laying iron track will be a challenge.
Re-doing that section of track would be expensive AND a traffic nightmare.
That section of I-5 is congested enough already. First problem is its proximity to JBLM. Second is that this section is on a hill coming up out of the Nisqually river .
To do it right will take years, and drivers in the area will scream like stuck pigs if there are more delays on I-5.