Agile development is getting increasing attention for the development of systems - also in context of safetycritical applications. However, the assurance of emerging properties of a system like reliability or functional safety becomes an even more challenging task, since the system needs to be re-assessed in terms of reliability or safety in each development cycle (or sprint). Because the architecture as the basis for the reliability or safety analysis is continuously changing, a lot of extra work for maintaining the reliability/safety assessments is necessary by the experts. With Component Fault Trees (CFTs) there is an approach to cope with this challenge by a modular and hierarchical methodology to assess the safety or reliability of the system. In this paper, we show the application of the CFT methodology in an iterative and incremental development life-cycle. Using a case study in the railway domain we demonstrate that the CFT methodology eases the reliability/safety assessment in agile development significantly.