These are mentioned in the book American Narrow Gauge Railroads, and may not all be narrow gauge.
- Addison and Pennsylvania Railway
- Ahukini Terminal and Railroad Company
- Akron, Canton and Youngstown Railway
- Alabama, Tennessee and Northern Railroad
- Alabama Coal, Iron and Railroad Company
- Alabama Midland Railway
- Alabama Mineral Railroad
- Alabama and Mississippi Railroad
- Alberta Railway and Coal Company
- Alberta Railway and Irrigation Company
- Alden Bridge and Camden Railway
- Allegheny Valley Railroad
- Alliance and Lake Erie Railroad
- Alliance and Northern Railroad
- Alpena and Northern Railroad
- Altoona, Clearfield and Northern Railroad
- Altoona, Juniata and Northern Railway
- Altoona Northern Railroad
- Altoona and Phillipsburg Connecting Railroad
- Altoona and Wapsononock Railroad
- American Central Railway
- American Fork Railroad
- Americus, Preston and Lumpkin Railroad
- Angelina and Neches River Railroad
- Ann Arbor Railroad
- Anniston and Atlantic Railroad
- Anniston and Cincinnati Railroad
- Antofagasta Railway
- Arcade and Attica Railroad
- Arcadia and Betsey River Railway
- Arcata Transportation Company
- Arcata and Mad River Railroad
- Argentine Central Railway
- Argentine and Gray's Peak Railway
- Arizona Eastern Railroad
- Arizona Narrow Gauge Railroad
- Arizona and New Mexico Railway
- Arkansas Central Railway
- Arkansas City and Grenada Railroad
- Arkansas Midland Railroad
- Arkansas Southwestern Railroad
- Arkansas and Indian Territory Railway
- Arkansas and Louisiana Railway
- Arlington and Atlantic Railway
- Aroostook River Railroad
- Aspen and Western Railway
- Atchison, Topeka and Santa Fe Railway
- Atlanta, Knoxville and Northern Railway
- Atlanta and Birmingham Air Line Railway
- Atlanta and Charlotte Air Line Railway
- Atlanta and Florida Railroad
- Atlanta and Hawkinsville Railroad
- Atlanta and Richmond Air-Line Railway
- Atlanta and Roswell Railroad
- Atlanta and West Point Railroad
- Atlantic City Railroad
- Atlantic Coast Line Railroad
- Atlantic and Danville Railway
- Atlantic and Great Western Railway
- Atlantic and St. Lawrence Railroad
- Atlantic and Yadkin Railway
- Atlantic-Pacific Tunnel and Grays Peak Railway
- Atlin Tram
- Attica and Allegheny Valley Railroad
- Attica and Arcade Railroad
- Attica and Freedom Railroad
- Au Sable and Northwestern Railroad
- Augusta, Gibson and Sandersville Railroad
- Augusta Southern Railroad
- Augusta and Southeastern Railway
- Aurora, Elgin and Chicago Railway
- Austin City Railway
- Austin and Northwestern Railroad
- Austin and Southeastern Railway