  Freight train services in a railway network system are generally divided into
two categories: one is the unscheduled train, whose operating frequency
fluctuates with origin-destination (OD) demands; the other is the scheduled
train, which is running based on regular timetable just like the passenger
trains. The timetable will be released to the public if determined and it would
not be influenced by OD demands. Typically, the total capacity of scheduled
trains can usually satisfy the predicted demands of express cargos in average.
However, the demands are changing in practice. Therefore, how to distribute the
shipments between different stations to unscheduled and scheduled train
services has become an important research field in railway transportation. This
paper focuses on the coordinated optimization of the rail express cargos
distribution in two service networks. On the premise of fully utilizing the
capacity of scheduled service network first, we established a Car-to-Train
(CTT) assignment model to assign rail express cargos to scheduled and
unscheduled trains scientifically. The objective function is to maximize the
net income of transporting the rail express cargos. The constraints include the
capacity restriction on the service arcs, flow balance constraints, logical
relationship constraint between two groups of decision variables and the due
date constraint. The last constraint is to ensure that the total transportation
time of a shipment would not be longer than its predefined due date. Finally,
we discuss the linearization techniques to simplify the model proposed in this
paper, which make it possible for obtaining global optimal solution by using
the commercial software.
