Abstract: In the tractor-and-semitrailer transportation, a tractor does not need to wait for its trailer's cargo handling, therefore, the tractor-and-semitrailer transportation has high efficiency, and has seen gradually application in express transportation enterprises. This study is based on a mainline/subline transport network, and a new mode is proposed which is "midway trailer-uncoupling". Additionally, mainline tasks and subline tasks can be conducted by the same tractor. The problem is formulated as a mixed integer programming and a hybrid simulated annealing algorithm is adopted to solve it. In the algorithm, the initial feasible solution is constructed according to the principle of minimizing the distance between two consecutive tasks, and four kinds of neighborhood search methods are used to improve the diversity of solutions. Under the two kinds of simulation scenarios, changing the proportion of mainline/subline tasks and changing the total number of tasks, the total cost of our tractor-and-semitrailer transportation mode is compared with those of the other two modes in a total of 19 numerical examples. The other two modes refer to "no midway trailer-uncoupling, mainline tasks and subline tasks can be conducted by the same tractor" and "no midway trailer-uncoupling, mainline tasks and subline tasks must be separately conducted by different tractors". The results show that our mode is the optimal in all the numerical examples. As for the comparison of the other two modes, 1) when the total number of tasks changes, the mode "no midway trailer-uncoupling, mainline tasks and subline tasks can be conducted by the same tractor" is always superior to the other mode, "no midway trailer-uncoupling, mainline tasks and subline tasks must be separately conducted by different tractors"; 2) when the proportion of mainline/subline tasks changes, the former mode is more suitable for the cases with smaller proportion of subline tasks, the latter mode is just the opposite. The results of the numerical examples verify the validity of the mathematical model as well as the obvious advantage and robustness of our mode, and provide decision support for express transportation enterprises' actual operation.

Key words: tractor-and-semitrailer transportation, mainline/subline transport network, midway trailer-uncoupling, mainline/subline tasks, simulated annealing algorithm