- the fifth wheel position on the lead trailer of a B Train must not be located more than 0.3. meters behind the center of the last axle on the lead semitrailer. Weight Limits: - the steering axle weight limit on straight trucks was increased to 7250 kg. - weight limit caps on the second trailer of A and C train double trailer combinations were. removed and replaced by the requirement that the weight of the tractor drive axles + the.