The " Nisqually Line " is actually comprised of portions of the former Northern Pacific Prairie Line, and their American Lake Line, between Tacoma and Nisqually,WA. The line connects to the current BNSF Point Defiance Line at both ends, using a small amount of the former Milwaukee Road Tacoma Gulch line as well. ( See Steve Carter's post above for more info. ) The whole idea is to remove Amtrak passenger trains from the Point Defiance line, with its' bottleneck single track tunnels at Ruston & Point Defiance. These tunnels were originally double - track, but had to be single - tracked ( in the 1970's ??? ) to allow for longer, taller auto racks which used to be routed via the Prairie Line from Tenino to Tacoma.
Personal opinion : While BNSF & Amtrak are bragging up the time savings for Amtrak via the Nisqually Line ( a whopping five minutes ), I believe BNSF simply wants Amtrak off their main freight line. Amtrak, WSDOT, and Sounder are spending big taxpayer bucks to upgrade the Nisqually Line for this five minute savings, which will benefit BNSF much more than Amtrak, as freight trains will no longer have to hold at either end of the tunnels for those pesky Amtrak trains, only for other freights. Meanwhile, Amtrak's Coast Starlight will lose even more of its' coast running.