There aren't many trails that go over 495. This one also goes over 270!
The Bethesda Trolley Trail is a great urban run that mixes paved trails, major highway crossings via pedestrian bridges and a bit of street running to link them all together. The cool thing about this run is that you go right through the heart of Bethesda.
There are many ways to lengthen this run as you head past Georgetown Prep School, Fleming Park and the National Institutes of Health campus -- each has some additional paths/trails that you can use to get in more miles, if you are interested.
Since this is a straight-line trail, you'll need to make accommodations to get back to your starting spot or else go to a landmark and turn around.