A single-axle tractor pulling a long trailer faces an elevated risk of getting stuck on a railroad crossing when which condition exists?

AAMVA CDL Manual, Section 6.1.4 'Railroad-highway Crossings,' p. 6-2

This is a specific, non-obvious equipment mismatch that a driver would not notice just by looking at the trailer, since the landing gear height was set for a different tractor type. Recognizing this configuration risk helps a driver choose an alternate route or adjust landing gear before approaching a raised crossing.