What led to merger of Railway and Union Budget in 2017?

Railway and Union Budgets were presented separately until 2017 when government merged them. It was done after discussions between then minister Suresh Prabhu and then Finance Minister Arun Jaitley on NITI Aayog's white paper 'Dispensing with the Railway Budget'. Jaitley raised the issue in parliament, which was told a unified Budget would reflect holistic image of government's financial position.