40 CFR §201.28

1. (a)To determine whether it is probably complying with the regulation, and therefore whether it should institute noise abatement, a railroad may take measurements on its own property at locations that:
   1. (1)Are between the source and receiving property
   2. (2)Derive no greater benefit from shielding and other noise reduction features that does the receiving property; and
   3. (3)Otherwise meet the requirements of § 201.25.
2. (b)Measurements made for this purpose should be in accordance with the appropriate procedures in § 201.26 or § 201.27. If the resulting level is less than the level stated in the standard, then there is probably compliance with the standard.
3. (c)This procedure is set forth to assist the railroad in devising its compliance plan, not as a substantive requirement of the regulation.