Dave,
First, let me offer a "disclaimer" and suggestion: I have never really enjoyed Jacksonville as a train-watching mecca. Instead, I find the "outlying" portions of Jacksonville - Duval County as being much more enjoyable train-watching locations: Such as Baldwin and Callahan, FL
BALDWIN, FL:
Baldwin is located west of downtown Jax via I-10 or U.S. 90. From I-10, exit on U.S. 301 interchange south. Two blocks south, turn left onto a road beside the McDonalds, and proceed left into CSX's Baldwin Yd entrance road, and north to the Yard Tower. A fair amount of CSX + other road's power is lined up for assignments. STAY OUT OF THE WAY!
A short distance north is a major CSX interlocking, that can be reached from U.S 310 north: a right turn on Oliver St. From U.S. 90: south on S. Center St. Eastbound CSX freights often tie up west of the U.S. 301 crossing.
CALLAHAN, FL:
The small city of Callahan is located about 30 minutes northwest of downtown Jacksonville on U.S. 1. Callahan is the southern end of the "Folkston Funnel". and the junction point for the CSX mainlines into Jacksonville and to Baldwin Yard. From U.S. 1, turn left (west) onto Brandies Ave at the traffic light, and continue to the RR tracks.
JACKSONVILLE, FL:
If you must, then here are some good spots:
1. CSX's Jacksonville Intermodal Terminal =
Take I-295 north from I-10 or south from U.S. 1 or I-95 on the north side of town. Exit I-295 west on Prichard Rd., and go west to the overpass that is visible from the interchange. The overpass spans CSX's Intermodal Terminal. Best vehicle parking on w. side of the overpass. Sidewalk views from overpass (thru protective wire - bummer!).
A key location for other good train-watching spots is the intersection of U.S. 1 and Edgewood Avenue (S.R. 111) on the NW side of town.
From this intersection location:
NS's Simpson Yard area:
Go west on S.R. 111 about 1 block, and turn Right onto Edgewood Rd, which will take you through a large cemetery. Continue on and you will cross the CSX's double main line, a block later, you will cross the single track NS main line at the N end of Simpson Yard. Continue to another intersection with Edgewood Ave (SR 111). Turn R and go 1 block to the Old Kings Road intersection (traffic light). Turn L on Old Kings Rd and go to 2nd RR crossing. A gravel pull-off is on Left side, across the tracks from NS's Simpson Yd Office and loco servicing/car repair area at the S. end of NS's Simpson Yd.
Amtrak Station:
Go south on US 1 to Clifford Lane, turn right and continue to the Amtrak Station.
CSX Moncrief Yard area:
I am not recommending that you seek out CSX's Moncrief Yd. The roadways to and in the Yard are private property, and are well-patrolled. The local roads in the vicinity of the Yard, especially the south end, are in very unsafe areas, and your personal safety is definitely at risk! This area is definitely not where I want to spend the last moments of my life!
Talleyrand Docks area:
The Genesee & Wyoming operates the Talleyrand Terminal RR, and has 3 locomotives posted in this area for local switching services. The locos are usually parked a short distance to the W. of Tallyrand Blvd.
To reach this location, take US 1 east of I-95 north of downtown, exit US 1 at 8th Street interchange. Head E on 8th St to the intersection with Tallyrand Blvd, turn L (north) on Tallyrand to railroad crossing, and look for locomotives parked adjacent to a paved parking lot. on w. side of Tallyrand Blvd.
Now, my best suggestion is that you take the time (approximately one hour travel time on U.S. 1 each direction from downtown Jacksonville) to travel north to Folkston, GA. Folkston, GA has a railfan's viewing platform for watching the CSX trains on the "Folkston Funnel". The platform is covered, with ceiling fans, chairs, picnic tables, BBQ grills and restrooms, and has a speaker for listening in on train communications with the dispatchers in Jacksonville. Overhead fLoodlights come on at darkness for night-time illumination of passing trains, enabling train watchers to film the passing trains on a 24 - 7 basis. Folkston is a very friendly town, very safe, and has lots of amenities.
I live in the Orlando, FL area, and I usually bypass Jax (except for Baldwin or FEC's Bowden Yd) to spend quality time at Folkston, and Waycross, GA. Also, less than 1 hour north of Folkston is Waycross, GA, the home of CSX's giant Rice Classification Yard and major repair shops. Again, lots of action, and good, safe places to watch it.
I have posted instructions to FEC locations in Jax. on your FEC info. request.
Good luck and great train-watching!
ACLfan