49 CFR §219.505

An applicant who has tested positive or refused to submit to pre-employment testing under this section may not perform regulated service for any railroad until he or she has completed the Federal return-to-duty process under § 219.104(d). An applicant may also not perform DOT safety-sensitive functions for any other employer regulated by a DOT agency until he or she has completed the Federal return-to-duty process under § 219.104(d). This section does not create any right on the part of the applicant to have a subsequent application considered; nor does it restrict the discretion of the railroad to entertain a subsequent application for employment from the same person.