The New Mexico Rail Runner is 96.5 miles, 74.2 of which was preexisting BNSF track which is still shared with freight and also I believe the Amtrak Southwest Chief. There are some other segments that are also preexisting track, but they are much smaller and some was improved as part of the project so I won't even try to figure out what proportion was what there.

So strictly less than 22.3 miles of new track. For $385 million.