Rules for testing the brakes of each railway car when transferred to another train engine.
quote....A transfer train, as defined in § 232.5, shall receive a brake test performed by a qualified person, as defined in § 232.5, that includes the following:

(1) The air brake hoses shall be coupled between all freight cars;

(2) After the brake system is charged to not less than 60 psi as indicated by an accurate gauge or end-of-train device at the rear of the train, a 15-psi service brake pipe reduction shall be made; and

(3) An inspection shall be made to determine that the brakes on each car apply and remain applied until the release is initiated by the controlling locomotive. A car found with brakes that fail to apply or remain applied may be retested and remain in the train if the retest is conducted as prescribed in § 232.205(c)(4); otherwise, the defective equipment may be moved only pursuant to the provisions contained in § 232.15, if applicable; https://www.law.cornell.edu/cfr/text/49/232.215
quote...Class I brake test pursuant to §232.205
by a qualified mechanical inspector to
ensure 100 percent effective and operative brakes, a freight car inspection
pursuant to part 215 of this chapter by
an inspector designated under §215.11 of
this chapter, and all cars containing
non-complying conditions under part
215 of this chapter shall either be repaired or removed from the train. The
inbound inspection required by paragraph (a)(6) of this section may be used
to meet these inspection requirements
provided it encompasses all the inspection elements contained paragraphs
(a)(2) through (a)(4) of this section. https://www.govinfo.gov/content/pkg/CFR-2010-title49-vol4/pd...

Attempted trans...transferring railway cars that have been separated from a series of cars and drive engine that have passed a station going to the final destination, may proceed provided a complete brake system test has been carried out within the previous 24 hours.