Cool. Did you notice the view of the tracks on both sides of the LA River at one point? Suspect you would be well advised, though, to do that walk during the day in a group ... could get interesting and personally hazardous otherwise.

Turns out there are several spots around the state where odd things are practically on the freeway or in some other interesting (?) place. Marin's got a bunch of them. Consider also (from some educated Google Map browsing - certainly not exhaustive):

Bike path over the Yolo Causeway (careful, you might get run down by the racers): [www.google.com]

Path over Puerto Suelo Hill (San Rafael) - at one time, you had to ride/walk on the freeway shoulder here; there was an unofficial path along a drainage ditch and maintenance road but the modern path is a great improvement: [www.google.com]

Cal Park Hill path - you can get through the tunnel before SMART does: [www.google.com]

Path between Hamilton and Terra Linda: [www.google.com]

Paths tacked onto freeway ramps at Corte Madera Creek: [www.google.com]

Mill Valley-Sausalito path: runs along the former roadbed of NWP: [www.google.com]

There's an Arroyo Seco Bike Path in Google that runs mostly on the high-flow channel bench of the concrete flood control ditch (Arroyo Seco) followed by 110 betweeen LA and Pasadena. Probably pretty noisy, and not very scenic in spots. Though it might be fun to ride the Gold line up the hill (on the former Santa Fe alignment) then return down the bike path - unfortunately, there aren't any stations really near the bike path so some street commuting would be needed. Again, best to go during the day in a group ...

Looks like some interesting bike and hiking trails in the Rose Canyon area north of San Diego providing both railroad (Surfliner/Coaster) and freeway involvement - possible starting point might be [www.google.com]

und so weiter...