## 1 Introduction  
Due to growing concerns regarding global climate change and the need for sustainable development within transportation systems, there has been a consistent push to optimize transport resources to accommodate human and goods transportation. The recent focus has been on merging passenger and freight transportation, a concept that allows for space sharing in single vehicles, like buses, trains, and trams, for both passengers and freight. Frequently referred to as \"urban co-modality\" in various studies [1, 2, 3, 4, 5], this solution has the potential to decrease the demand for freight transport vehicles, ease traffic congestion, enhance service vehicle utilization rates, and ultimately provide environmental benefits such as reducing emissions [6, 7, 8, 9]. Co-modality has long been applied in the transportation sector for decades. Early applications can be found in long-haul transportation modes with sufficient capacities, such as air, rail, and waterborne transportation. However, realizing co-modality via other urban transport modes, such as transit and private vehicles, has not received enough attention until recent years. For example, DHL piloted a system in Schwerin, Germany, where trans and vans were combined to reduce traffic congestion and emissions [10]. Similarly, the Vert-Che-Voux system in Paris utilizes boats and bikes to deliver parcels, demonstrating how alternative modes of transport can complement each other in dense urban areas [11]. Another notable example is Daimler's concept of combining vans and drones, which aims to enhance last-mile delivery flexibility in hard-to-reach locations [12]. In China, major cities like Beijing and Shenzhen have also launched pilot schemes of integrating passenger and parcel movements via metro systems [13]. As evidenced by the previous studies, introducing co-modality in urban transit systems could bring additional benefits, including reliability and ridership enhancement [14; 15; 16], extra revenue [17], and more employment [18]. Moreover, urban transit services are ideally suited for implementing co-modality because of their large transport capacities and established infrastructure, such as bus and metro stations, which enhance their potential for mass freight transport, while maintaining low construction and operating costs. Despite the advantages of co-modality in transit services, challenges arise as most transit lines are fixed or constructed underground (e.g., metro lines), preventing transit vehicles from performing last-mile deliveries. In city logistics, last-mile deliveries refer to the final step in the logistics process, where goods are transported from a distribution center or warehouse to the end customers. This segment is particularly crucial in city logistics due to its complexity and high costs, which can account for up to 53% of the total shipping expenses [19]. Therefore, additional service providers are needed to complete last-mile delivery in transit co-modal systems. As suggested by the previous studies and practical applications, these service providers can be (1) crowdsourced workers who transport freight on foot or by non-motorized vehicles, such as shared bikes and e-scooters [20]; (2) taxis and ride-sourcing vehicles [21]; and (3) robots or autonomous vehicles [22; 23; 24; 25]. Among these last-mile delivery service providers, the ride-sourcing vehicles, which provide flexible door-to-door and prompt ride services to travelers, have shown their great potential in reshaping the way we travel. Like the transit co-modality system, one possible way for the ride-sourcing vehicles to perform last-mile delivery is to integrate last-mile delivery with passengers' trips, leading to a so-called _ride-pooling service with passenger-parcel integration_ (herein, we use \"parcel\" to refer to freight). Such integration can yield several advantages. Firstly, it can increase the utilization rate of ride-sourcing vehicles. Secondly, it might make ride-pooling services with parcels more appealing to passengers.