The convention, familiar with cabooses, defines the B end as the end which has the direct connection to the piston of the brake cylinder (as modelers say, the cylinder "points" to the B end). With a second brake wheel, of course, levers provide an INDIRECT connection to the cylinder too. Since not everyone knows the convention, or can readily see the brake cylinder, such cars often have the A and B ends indicated with stencils, as Tim O'C pointed out.