Huntsville is served by Norfolk Southern, the Huntsville & Madison County Railroad Authority, the International Intermodal Center, and the Mercury & Chase Railroad.
While Huntsville is in Alabama, the NS tracks belong to the Tennessee division, not the Alabama Division; the main line runs east to west, from Chattanooga to Memphis (in Sheffield, a line runs south to Birmingham and is part of the Alabama Division). But, Norfolk Southern is not the only railroad in the area. Operating over ex-L&N track is the Huntsville and Madison County Railroad Authority; a shortline serving a few industries south of Huntsville and interchanging with NS in downtown Huntsville. Also operating over ex-L&N trackage is the Mercury & Chase Railroad, run by the North Alabama Railroad Museum, a chapter of the National Railway Historical Society. The Airport Authority runs the International Intermodal Center on tracks that were built for that purpose.
Decatur is an interesting railroad town. The city is served both by Norfolk Southern and by CSX. These two class one giants run over the same drawbridge over the Tennessee River (the CSX line runs north to south, the NS line runs east to west). The drawbridge is owned by NS. Both railroads have small yards in Decatur and the drawbridge is protected by a modern tower.
Between Huntsville and Decatur, lies the town of Madison. Madison is the closest town to the Huntsville International Airport; an airport that operates the International Intermodal Center, operating two switchers and a giant crane.
Someday I (dk) hope to get maps up of the area. Photography of trains in Huntsville is difficult in most places, as the tracks are often hard get to and view. Decatur has some good places to get photos of the drawbridge, mainlines, and industrial sidings. The International Intermodal Center is fenced off, but the switchers operate right next to the highway, so getting photo's is possible (obviously you shouldn't tresspass anywhere, but if you are going to, don't do it at the IIC--the United States Customs Service has offices at the IIC, as well as being patrolled by Airport Police).
The NS line sees 10 to 20 trains a day--many comming at night for poor viewing opportunities. The CSX line sees about twice as many (don't know specifics--anyone who does, please write). The IIC operates at least 6 days a week (maybe 7) and operates at least durring regular business hours (maybe 24 hours). The H&MRR (aka H&MC, aka H&MCRrA) operates on an 'as needed' schedule, a few times a week (usually twice a week).
The North Alabama Railroad Museum runs the Mercury & Chase Railroad over 5 miles of track. The line is former L&N, and was originally NC&StL.
Here are some pics of the Huntsville Depot, now a museum.