49 CFR §221.3

1. (a)Except as provided in paragraph (b) of this section, this part applies to passenger, commuter and freight trains when operated on a standard gage main track which is part of the general railroad system of transportation.
2. (b)This part does not apply to:
   1. (1)A railroad that operates only trains consisting of historical or antiquated equipment for excursion, educational, or recreational purposes;
   2. (2)A train that operates only on track inside an installation which is not part of the general railroad system of transportation;
   3. (3)Rapid transit operations in an urban area that are not connected with the general railroad system of transportation.
   4. (4)A railroad that operates only one train at any given time.