## 4 Logistics and Cold Transportation in Future Green Cities
### Underground and Co-Modal Urban Logistics Systems  
In areas of great population growth and high demand for new living space, whole new cities and urban areas are being built from scratch. While old cities of the world are universally vexed by the planning constraints placed on them by infrastructure and zoning made for a bygone era, a modern, newly built city has no such constraints. In China, as new cities are being built, we are seeing a shift in research towards underground logistics systems (ULSs) [50]. The excavation of predominant ULS pathways--to act as a city's veins and arteries for goods--is significantly easier and less costly when there is not yet above-ground or other below-ground infrastructure impeding the construction. The shape of the new city can be planned to support the ULS, meaning systems like this, although still challenging to implement, become more viable economically. Although the interest in and demand for ULSs is growing, planning and constructing them still poses a great challenge, and there is little data on their implementation [50]. An especially promising type of ULS for future green cities is the metro-based ULS (M-ULS), which is also a promising form of co-modal--that is, passenger combined with freight--urban transport. In a M-ULS, the underground infrastructure is predominantly rail--as opposed to, for example, road traffic tunnels--and works to synergistically transport both passengers and goods within the city. Not only has this metro system been shown to have high potential for freight, but a modern M-ULS system would aid a city in all areas of sustainability--economic, social, and environmental [51]. An existing city's metro network can also be transformed into a M-ULS, but the network will need to be expanded in order to guarantee that it acts as a functional hybrid transportation system [51]. Systems like M-ULS need custom optimization methods. As M-ULS infrastructure is being built and expanded, ways to calculate the best shape and structure for the network are needed. One method for M-ULS planning first evaluates the underground freight volume, calculates the optimal location-allocation of nodes, and lastly utilizes a hybrid of E-TOPSIS, the exact algorithm, and the heuristic algorithm to achieve the final result [52]. Another study uses a M-ULS network location-allocation-routing fuzzy random programming model with a crisp linearization method and carries out combinatorial optimization utilizing a discrete binary chaos particle swarm optimization-genetic algorithm in conjunction with exact algorithms [49]. A metro-based last-mile delivery system was also proposed by [PERSON] [53] using multiple unnamed custom algorithms, a genetic TSP algorithm, and a courier management algorithm. This shows that although research on M-ULS optimization exists, it is still scarce and dominated by Chinese research. A Chinese literature review [54] on the topic also found that, as it is, ULS and M-ULS research is still on a highly theoretical level, and rarely discusses aspects of engineering and application. Other types of co-modal urban transportation systems are also being researched, but they do not have nearly as much research effort being put into them as M-ULS, with both their variety and volume being quite low. For example, all short-haul co-modal transportation projects--the ones most relevant to urban transportation--ongoing in 2023 were tram projects [55]. Although the number of papers on co-modal tram transport is quite low, optimization and modeling frameworks for this form of operation still exist [56; 57]. Though the implementation of urban co-modal transportation systems is not without difficulty, a literary review of urban co-modality studies found that co-modality is held as a promising way to reduce congestion and pollution in city centers, and that it could synergistically benefit both freight and passenger transport, if done correctly [55].