The Elliot Bay Trail runs along the west side of the yard, from 20th Avenue to 16th Avenue. It offers great views of the hump, and locomotive storage next to 20th Avenue. If you continue up 20th Avenue, you can get decent views of the car shop, and roundhouse. The Dravus Street overpass also gives good views. It is probably best to first survey the area using satellite maps.
Oh, and through trains seem to pass by every half hour or so.