| 1 | NIA | Nordavia Fluggesellschaft mbH | Callsign: Nordavia, | D | AVCODES |
| 2 | NIA | Nile Air | IATA: NP, Callsign: Hapi Air, ICAO callsign changed to Nile Bird | ET | AVCODES |
| 3 | NIA | Nile Air | IATA: NP, Callsign: Nile Bird, ICAO callsign changed from Hapi Air | ET | AVCODES |
| 4 | NIAC | NIATCO TRUCKING CORP | | | SCAC3 |
| 5 | NIAG | Niederrheinische Verkehrsbetriebe AG | Moers | D | privat-bahn.de |
| 6 | NIAG | Niederrheinische Verkehrsbetriebe AG | | D | TRAFFIC |
| 7 | NIAG | Niederrheinische Verkehrsbetriebe | | DE | VKM |
| 8 | NIAJ | Consolidated Rail Corp. | 1/1980 | | NAKINA |
| 9 | NIAJ | Niagara Junction Ry. Co. | 1/1975 | | NAKINA |
| 10 | NIAJ | Niagara Junction Ry. Co. | Jan-75 | | NRHS |
| 11 | NIAJ | Consolidated Rail Corp. | Jan-80 | | NRHS |
| 12 | NIAJ | CONRAIL [NIAGARA JUNCTION RAILWAY] | | | PWRC |
| 13 | NIAJ | Niagara Junction Railway (Conrail) | | | RSRV |
| 14 | NIAL | NIAGARA LEASING CORP | | | SCAC3 |
| 15 | NIAR | NICHOLS & RONK INC | | | SCAC3 |
| 16 | NIAT | NATIONAL TRANSPORT SERVICES CO INC | | | SCAC3 |
| 17 | NIAT | NIAGARA TANK LINES INC | | | SCAC4 |
| 18 | NIAX | Niagara & Western New York R.R. Co., Inc. | 7/2005 (not listed) | | NAKINA |
| 19 | NIAX | Niacet Chemicals Division, United States Vanadium Corp. | 1/1946-1/1950 | | NAKINA |
| 20 | NIAX | Niacet Chemicals Corp. | 7/1930-1/1945 | | NAKINA |
| 21 | NIAX | *NIACET CHEMICALS DIV., US VANADIUM CORP. | | | PWRC |
| 22 | NIAX | *NIAGARA & WESTERN NEW YORK RAILROAD | | | PWRC |
| 23 | NIAX | Niagara and Western New York Railroad | | | RSRV |
| 24 | NIAX | Niagara & Western New York Railroad | | USA | TRAFFIC |
