Could you please provide a comprehensive and detailed explanation of the selection process for the RRB exams, outlining each stage involved from the initial application to the final selection? It would be helpful if you could break down the various phases of the process, such as the preliminary exam, main exam, document verification, and any other stages, and explain the role of each phase in determining a candidate’s eligibility. Additionally, could you describe the types of assessments or tests conducted at each stage, the selection criteria used by the Railway Recruitment Board, and how candidates are ranked based on their performance? Furthermore, it would be valuable to know if there are any specific requirements, such as medical tests or skill assessments, that candidates must undergo as part of the selection process, and how the final merit list is prepared. Lastly, could you explain the overall timeline for the entire selection process, from application submission to final appointment, and any other important factors candidates should consider to increase their chances of success?