(b) This part does not apply to:

1. (1) A railroad with track gage of less than 24 inches;
2. (2) A railroad that operates exclusively freight trains and does so only on track inside an installation that is not part of the general system of transportation;
3. (3) Rapid transit operations in an urban area that are not connected to the general system of transportation; or

4. (4) A railroad that operates passenger trains and does so only on track inside an installation that is insular, i.e., its operations are limited to a separate enclave in such a way that there is no reasonable expectation that the safety of the public—except a business guest, a licensee of the railroad or an affiliated entity, or a trespasser—would be affected by the operation. An operation will not be considered insular if one or more of the following exists on its line:

   1. (i) A public highway-rail crossing that is in use;
   2. (ii) An at-grade rail crossing that is in use;
   3. (iii) A bridge over a public road or waters used for commercial navigation; or
   4. (iv) A common corridor with another railroad, i.e., its operations are conducted within 30 feet of those of any other railroad.