  This study explores the potential of using public transportation systems for
freight delivery, where we intend to utilize the spare capacities of public
vehicles like buses, trams, metros, and trains, particularly during off-peak
hours, to transport packages within the city instead of using dedicated
delivery vehicles. The study contributes {to the growing} literature on
innovative strategies for performing sustainable last mile deliveries. We study
an operational level problem called the Three-Tier Delivery Problem on Public
Transportation, where packages are first transported from the Consolidation and
Distribution Center (CDC) to nearby public vehicle stations by delivery trucks.
From there, public vehicles transport them into the city area. The last leg of
the delivery is performed to deliver the packages to their respective customers
using green vehicles or eco-friendly systems. We propose mixed-integer linear
programming formulations to study the transport of packages from the CDC to the
customers, use decomposition approaches to solve them, and provide numerical
experiments to demonstrate the efficiency and effectiveness of the system. Our
results show that this system has the potential to drastically reduce the
length of trips performed by dedicated delivery vehicles, thereby reducing the
negative social and environmental impacts of existing last mile delivery
systems.
