Urban transportation systems' structure and functionality can be affected by unexpected disruptions due to several reasons, like as natural hazards, intentional attacks, accidents, etc. Resilience is generally understood as the ability to recover from a disruptive event. There are multiple studies in the literature focusing on resilience assessment and improving the resilience level of mobility services prior to disruption. However, few studies offer solutions for transportation systems' operators during disruptions to reduce their impacts (e.g., recovery time). In this context, a new paradigm, called Resilience as a Service (RaaS), has emerged in the field of operation management. The idea of RaaS is to integrate the resource management system of different service providers to improve the resilience level of the entire system and provide robust solutions for disruption management. This paper proposes a definition of RaaS dedicated to transportation systems. To provide a methodological example corresponding to the RaaS paradigm, we formulate a bi-level optimization problem to represent a solution example that RaaS providers can deliver. The upper-level model formulates the resource reallocation problem during disruption from the perspective of RaaS providers, while the lower-level model considers user perspectives by calculating user equilibrium, i.e., minimizing the travel cost of every user. We provide a numerical example in a real test case of a French city to illustrate the benefits of implementing a RaaS solution. The results show that we can reduce the average travel delay of all users by 69% compared to the absence of RaaS.