My son goes to college in G'burg, so I've spent a little time railfanning the yard, which is one of the biggest in the country, I believe. Start by visiting the RR Museum (next to the Amtrak station); they had a little map showing the best locations for railfanning. My best places:
1) By the Museum. 2) A couple of miles south of there is an east-west road (from Mapquest, it looks like KNox Hwy 10) which leaps over the yard. The bridge is four lanes wide, so you can park on it and just watch the yard at your leisure (note: this is the *second* overpass south of the Amtrak station. The first one is only two lane, and in a not-so-nice neighbourhood). 3) Gravel access road on west side of yard, where you can see the top of the hump without going on the property. 4) The E-W line on the north side of Main Street gets....lots of trains.Basically, Galesburg is train heaven ;-).
Rochelle? Also a good place.
-- Kizhe