yup BRAS stands for broadband remote access server..the difference between MTNL mumbai and MTNL delhi is that mumbai has distributed BRAS setup while in delhi all the remote access servers are located at one place i.e Kidwai Bhawan, Janpath..dont know if this difference has any effect on reliablility though..
It does have an effect on reliability because distributed systems like these are inherently fault-tolerant as in if one node fails, all of them don't go down, provided the nodes can function independently.