| 5101 | SIRQ | SCHNEIDER TRANSPORTATION | | | SCAC4 |
| 5102 | SIRR | Southern Industrial R.R., Inc. | 1/1975-7/2005 | | NAKINA |
| 5103 | SIRR | Southern Industrial R.R., Inc. | 1/75-7/2000 | | NRHS |
| 5104 | SIRR | SOUTHERN INDUSTRIAL RAILROAD, INC. | | | PWRC |
| 5105 | SIRR | Southern Industrial Railroad | | | RSRV |
| 5106 | SIRR | SIERRA TRANSPORT | | | SCAC4 |
| 5107 | SIRS | STANDARD IRON METALS CO | | | SCAC4 |
| 5108 | SIRT | STATEN ISLAND RAPID TRANSIT OPERATING AUTHORITY (STATEN ISLAND RAILWAY) | | | PWRC |
| 5109 | SIRT | Staten Island Rapid Transit | City: New York | USA | TRAFFIC |
| 5110 | SIRU | OOO VED CONTACT | | RUS | BIC |
| 5111 | SIRX | Southern Illinois Railcar Co. | added 1/1991; 1/1994-7/2005; eliminated 1/2008 | | NAKINA |
| 5112 | SIRX | SC Rail Leasing America, Inc. d/b/a Southern Illinois Railcar Co. | added 1/2008 | | NAKINA |
| 5113 | SIRX | Southern Illinois Railcar Co. | added 1/91; 1/94-7/2000 | | NRHS |
| 5114 | SIRX | SC RAIL LEASING AMERICA, INC. d/b/a SOUTHERN ILLINOIS RAILCAR CO. | | | PWRC |
| 5115 | SIRX | Southern Illinois Railcar Corporation | | | RSRV |
| 5116 | SIRX | VTG RAIL INC | | | SCAC4 |
| 5117 | SIRX | Southern Illinois Railcar Company | | USA | TRAFFIC |
| 5118 | SIRY | Staten Island Ry. Corp. (transferred to New York, Susquehanna and Western Ry. Corp.) | added 7/1985; eliminated 10/1988 | | NAKINA |
| 5119 | SIRY | Staten Island Ry. Corp. | eliminated 4/1992; 1/1994 | | NAKINA |
| 5120 | SIRY | Staten Island Ry. Corp. (transferred to New York, Susquehanna and Western Ry. Corp.) | eliminated 10/88 | | NRHS |
| 5121 | SIRY | Staten Island Ry. Corp. | eliminated 4/92; 1/94 | | NRHS |
| 5122 | SIRY | RETIRED 02-13-92 ATP (0389) [STATEN ISLAND RY.] | | | PWRC |
| 5123 | SIS | Saber Airlines | | ET | AVCODES |
| 5124 | SIS | Scott Aviation, LLC d/b/a Silver Air | Callsign: Silver Speed, | USA | AVCODES |
| 5125 | SIS | Saber Airlines | | | ICAO1 |
| 5126 | SISB | SISTEMAS ECOLOGICOS PARA LA PROTECC | | | SCAC4 |
| 5127 | SISD | SIS | | | SCAC4 |
| 5128 | SISFC | SISTEMAS Y MONTAJES INDUSTRIALES, S.A. | | ES | VKM |
| 5129 | SISG | SARNIA INSULATION SUPPLY (A DIVISIO | | | SCAC4 |
| 5130 | SISI | SILVER SITE TRANSPORT LTD | | | SCAC4 |
| 5131 | SISJ | SOMERS ISLES SHIPPING LTD | | | SCAC1 |
| 5132 | SISJ | SOMERS ISLES SHIPPING LIMITED | | | SCAC4 |
| 5133 | SISK | SISTAD TRUCKING | | | SCAC4 |
| 5134 | SISL | SKIT INDUSTRIAL S DE RL DE CV | | | SCAC4 |
| 5135 | SISM | SUPREME INTERNATIONAL SHIPPERS AND | | | SCAC4 |
| 5136 | SISO | SISTEMAS DE TRAFICO INTERNACIONALES | | | SCAC4 |
| 5137 | SISR | SISSON TRANSPORTATION INC | | | SCAC4 |
| 5138 | SISU | ELBTAINER GMBH | | D | BIC |
| 5139 | SISU | SCANDINAVIAN CARGO SEA A.B. | | | PWRC |
| 5140 | SISU | Scandinavian Cargo Sea AB | | | RSRV |
| 5141 | SISU | TRANSAMERICA LEASING | | | SCAC4 |
| 5142 | SISW | SOLIS TRUCKING | | | SCAC4 |
| 5143 | SIT | SITA | IATA: XS | | ICAO1 |
| 5144 | SITAC | Calais Opale Bus | City: Calais | F | TRAFFIC |
| 5145 | SITB | SITSA | | | SCAC4 |
| 5146 | SITB | Sihltalbahn | | CH | TRAFFIC |
| 5147 | SITC | SKINNER TRANSFER CORP | | | SCAC1 |
| 5148 | SITC | SKINNER TRANSFER CORP | | | SCAC4 |
| 5149 | SITD | SIMBOL INTERNATIONAL TRADE | | | SCAC4 |
| 5150 | SITFA | Societa Italiana Trasporti Ferroviari Autoveicoli spa | | I | TRAFFIC |
| 5151 | SITFA | SITFA S.p.A. (TO) | | IT | VKM |
| 5152 | SITI | SIMON TRUCKING, INC | | | SCAC1 |
| 5153 | SITL | SAHAULI TRANSPORT LTD | | | SCAC4 |
| 5154 | SITM | SITTON MOTOR LINES INC | | | SCAC4 |
| 5155 | SITO | SITSA | | | SCAC4 |
| 5156 | SITP | SILICA TRANSPORT INC | | | SCAC4 |
| 5157 | SITQ | SITRA | | | SCAC4 |
| 5158 | SITR | SEVENSKI, STEPHEN TRUCKING INC | | | SCAC4 |
| 5159 | SITS | SIDNEY TRUCK & STORAGE, INC | | | SCAC1 |
| 5160 | SITT | SPIRIT INTERNATIONAL TRANSPORT INC | | | SCAC4 |
| 5161 | SITU | SITC CONTAINER LINES CO LTD | | PRC | BIC |
| 5162 | SITU | SITC CONTAINER LINES CO LTD | | | SCAC4 |
| 5163 | SITV | SONA INTER TRANSPORT INC | | | SCAC4 |
| 5164 | SITW | SCHISLER TRUCKING | | | SCAC4 |
| 5165 | SITWN | Siemens AG Transformatorenwerk Nürnberg | | DE | VKM |
| 5166 | SITX | Sherritt International Corp. | added 4/1997; 7/2000-7/2005; eliminated 1/2007 | | NAKINA |
| 5167 | SITX | Sherritt International Corp. | added 4/97; 7/2000 | | NRHS |
| 5168 | SITX | SHERRITT INTERNATIONAL CORPORATION | | | PWRC |
| 5169 | SITX | Sherritt International Corporation | | | RSRV |
| 5170 | SITY | SICC TRUCKING | | | SCAC4 |
| 5171 | SITZ | SITTON MOTOR LINES INC | | | SCAC4 |
| 5172 | SIUL | SCHULTE INDUSTRIES LTD | | | SCAC4 |
| 5173 | SIUU | CP SHIPS | | | SCAC4 |
| 5174 | SIV | Slovenian Armed Forces | Callsign: Slovenian, | SLO | AVCODES |
| 5175 | SIV | Slovenian Armed Forces | | | ICAO1 |
| 5176 | SIVA | SILVA HERNANDEZ, MARIA DE JESUS | | | SCAC4 |
| 5177 | SIVD | SIEVERS, SCOTT TRUCKING INC | | | SCAC4 |
| 5178 | SIVE | SIEVERS TRUCK SERVICE | | | SCAC4 |
| 5179 | SIVM | SILVICOM | | | SCAC4 |
| 5180 | SIVP | S P I INTERNATIONAL | | | SCAC4 |
| 5181 | SIVS | SIDER NAVI SPA | | | SCAC4 |
| 5182 | SIVW | SEAWATER INVESTMENT CORP | | | SCAC4 |
| 5183 | SIW | Sirio Executive S.r.l. | Callsign: Sirio Executive, | I | AVCODES |
| 5184 | SIW | Sirio Executive | | | ICAO1 |
| 5185 | SIWN | S H I INC | | | SCAC4 |
| 5186 | SIX | Sixt Rent A Car, LLC | Callsign: Drive Orange, | USA | AVCODES |
| 5187 | SIXA | SIX (6) FIGURED | | | SCAC4 |
| 5188 | SIXD | SIX EIGHT EIGHT SEVEN TWO SIX ONE ( | | | SCAC4 |
| 5189 | SIXH | SIX TWO FOUR ONE NINE FOUR EIGHT (6 | | | SCAC4 |
| 5190 | SIXL | SCARBROUGH INTERNATIONAL EXPRESS LI | | | SCAC4 |
| 5191 | SIXM | SIX ONE ONE ZERO SIX TWO TWO (61106 | | | SCAC4 |
| 5192 | SIXN | SIN EXPEDITING | | | SCAC4 |
| 5193 | SIXO | SIOUXLAND PROPANE CO | | | SCAC4 |
| 5194 | SIXP | SOUTHERN INTERMODAL XPRESS INC | | | SCAC4 |
| 5195 | SIXQ | SIX TWO SEVEN TWO ZERO NINE SIX (62 | | | SCAC4 |
| 5196 | SIXU | SOLVAY INDUPA DO BRASIL S A | | | SCAC4 |
| 5197 | SIXV | SIOUX VALLEY COOPERATIVE | | | SCAC4 |
| 5198 | SIXW | SIX (6) TRANS TRANSPORTATION INC | | | SCAC4 |
| 5199 | SIXX | 636 Leasing LLC | added 10/2003; eliminated 1/2004; 7/2005 (not listed) | | NAKINA |
| 5200 | SIXX | 636 LEASING LLC | | | PWRC |
