ethereal, I can see in Google Maps a really long railroad tunnel that begins at the intersection of I-5 and CA-14.
Check out this Google Street View link:
http://goo.gl/maps/C0pjA
Amid the maze of freeway ramps and pillars you can spot the tiniest glimpse of what appears to be a tunnel entrance, at a lower level than all of the surrounding roadways and pillars. If you switch to map view and zoom back out, you can follow the path of the rail line tunnel over a mile northward. There is a 45-degree aerial of the other end, but no street view. Metrolink's Antelope Valley Line runs through this tunnel toward Lancaster.