## 2 Literature Review  
As an environmentally friendly transportation mode in urban cities, bike sharing can improve the sustainability of urban transport [20]. After decades of practical and theoretical research, scholars have investigated the planning, operation, and management of bike sharing from different aspects, among which the usage pattern and operation characteristics are the basis of improving bike sharing systems. For example, [PERSON], [PERSON] and [PERSON] studied the spatial distribution of the bike-sharing usage patterns of five typical activities [21]. Due to the obvious fluctuations in bike-sharing operations, many studies focus on the rebalancing of bike-sharing. In the short term, the rebalancing depends on operators using vehicle fleets to move bikes between different locations. Previous studies proposed models for the pickup and delivery vehicle routing problem [22; 23]. In addition, user-based rebalancing operations were performed, classified as static and dynamic approaches [24; 25]. It is apparent that research on FFBS characteristics and rebalancing is limited compared with that on SBBS.  
With the development of urban rail transit systems in recent years, the combination of bike sharing and rail transit has become an important research focus. [PERSON] suggested that \"improved public transport integration\" be treated as an important research direction in the future when studying bike sharing [26]. [PERSON], [PERSON] and [PERSON] investigated the famous bike sharing system Velib and noted that the well-organized combination of bike sharing and rail transit could improve their use [27]. The research conducted by [PERSON] and [PERSON] observed that bike sharing had different influences on rail transit in different areas based on the development intensity. In an area with low development intensity, bike sharing mainly functions as a feeder mode for rail transit, whereas in dense urban cores, it can even replace rail transit by virtue of its advantages in mobility and speed, instead of functioning as a feeder of rail transit only [28]. [PERSON] et al. established mode choice models for public bicycles and private bicycles to quantify the influence of personal and station attributes on users who used public bikes as a feeder mode to rail transit. This study found that female, older, and low-income rail commuters are less likely to use a public bicycle to access rail transit [16]. [PERSON] et al. revealed the competition between BSS and car modes in general in dense urban areas [29]. [PERSON], [PERSON] and [PERSON] researched the influence of bike sharing on rail transit in Washington, D.C. and found that when the Cabi passenger flow increases by 10%, the passenger flow of the rail transit increases by 2.8% [30]. [PERSON] showed that bike sharing could relieve the congestion problem of rail transit compared with having on-board bicycles [31]. [PERSON] and [PERSON] researched the relationship between bike sharing and rail transit from the perspective of planning objectives and suggested a framework that can integrate them properly [5].  
Existing research has examined the operational features of FFBS, but few studies have investigated its operation as a feeder mode to rail transit. In this study, we endeavor to rectify this gap in literature. The analysis of FFBS operational characteristics as a feeder mode will assist researchers to further investigate the rebalancing and repositioning of FFBS and improve the management of urban public transport systems.