Which of the following is explicitly one of the four required topics on the S-endorsement knowledge test under 49 CFR 383.123(a)(2)?

49 CFR 383.123(a)(2)(iii)

Railroad-grade-crossing procedures are a frequently tested, high-stakes topic because crossing violations by school buses carry severe safety and liability consequences — knowing it's a codified, mandatory test topic signals how much weight to give it when studying.