As Bill mentioned, check the k8dti.com site - tons of great stuff including a railfan guide. I wouldn't recommend bringing a latter and don't get close to the tracks. The viaducts are also nice to check out the Queensgate yard.
Here's some recent pics from the Hopple St. Viaduct. (This was my first time really railfanning, so the pics aren't too great, but not bad!)
http://www.market-assist.com/~osterday/ ... pple-st-1/
As CIOR mentioned, across from NA tower, on Railroad Ave is a nice spot too.
As I'm getting ready to post this, I just realized the original post is from June 2005... oh well... maybe someone with still find this useful!