This chapter explores the notion of rail regeneration, a professional term evoking the idea of smart network modernization after decades of degradation and neglect. The theoretical notion (and the visible effects of its implementation) are subject to heated public debates because, in the context of a Paris mass transit line serving here as case study, the more the railway line is being regenerated, the less it seems to function satisfactorily. As the structural congestion of the Paris suburban network impedes a long closure, restorative works disturb considerably its routines. Consequently, the line seems to be neither broken nor functioning for an indeterminate and maybe interminable period. Our theoretical framework articulates two elements: the sociological concept of trial, which helps exploring the heterogeneous valuing processes underlying the notion of regeneration among engineers, institutional stakeholders and opinion; and a model of infrastructure resilience, which separates the process in two moments (disruption and recovery). Both tools enable us to consider the question - is regeneration doomed to fail? - as being composed of two different but intricated steps in this trial: one concerns the exact state of the line and the causal factors of its breakdown, the second concerns the capacity of regenerators to recover the railway line without destabilizing the whole mass transit socio-techno-political regime. Two results are being discussed. The first is that regeneration appears to be much more than an expert method apt to describe and fix the railway's vulnerability. As a reordering activity, it tries to stabilize an unstable arrangement of knowledges and powers and to ascertain the interests of the infrastructure manager during a rail reform that may marginalize its interests. But if regeneration means an effort to redefine and prioritize repair policies, it runs into too many metropolitan and transport regime obduracies. The second concerns the key importance of time in the design, conduct and lived experience of regenerative practices. As it permanently weaves past breakdown memories, painful present and future fears of infrastructural failure, the regeneration's promise (a horizon of infrastructure resilience) is constant risk of being replaced by and trapped into another mode of existence, the gloomy present of a now-and-for-ever-degraded suburban line. Hence, its promissory dimension as the solution to rail problems fades away with the eternalization of its intermediateness.