Which two conditions both let a bus driver skip the full stop at a railroad crossing, according to the CDL manual's exception list?

49 CFR 383.117(a)(4), Proper procedures at railroad-highway grade crossings and drawbridges

Memorizing the exact, narrow list of exceptions (not a broad category like "low traffic" or "bad weather") prevents drivers from wrongly assuming a stop isn't needed at an ordinary active crossing, which is one of the most common and dangerous mistakes tested on this part of the exam.