49 CFR §242.209

1. (a)Each conductor who has received a certificate required under this part shall:
   1. (1)Have that certificate in his or her possession while on duty as a conductor; and
   2. (2)Display that certificate upon the receipt of a request to do so from:
      1. (i)A representative of the Federal Railroad Administration,
      2. (ii)A State inspector authorized under part 212 of this chapter,
      3. (iii)An officer of the issuing railroad, or
      4. (iv)An officer of another railroad when serving as a conductor in joint operations territory.
2. (b)Any conductor who is notified or called to serve as a conductor and such service would cause the conductor to exceed certificate limitations, set forth in accordance with subpart B of this part, shall immediately notify the railroad that he or she is not authorized to perform that anticipated service and it shall be unlawful for the railroad to require such service.
3. (c)Nothing in this section shall be deemed to alter a certified conductor's duty to comply with other provisions of this chapter concerning railroad safety.