It most certainly is the law. It went into effect on Jan. 18, 2000, with the new steam rule (CFR 230). Read it yourself:
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ยง230.2 Applicability.
(a) Except as provided in paragraph (b) of this section, this part applies to all railroads that operate steam locomotives.
(b) This part does not apply to:
(1) A railroad with track gage of less than 24 inches;
(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) Rapid transit operations in an urban area that are not connected to the general system of transportation; or
(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:
(i) A public highway-rail crossing that is in use;
(ii) An at-grade rail crossing that is in use;
(iii) A bridge over a public road or waters used for commercial navigation; or
(iv) A common corridor with another railroad, i.e., its operations are conducted within 30 feet of those of any other railroad.
(c) See appendix A of part 209 for a current statement of the FRA's policy on its exercise of jurisdiction.
Sure, any company can build tracks across any road to link up their property. But remember that as soon as they do, they become non-insular. As I recall, the station crossing isn't the only one on the line. Any crossing that the general public has access to makes the railroad non-insular.