Most welcome step, using vertical space for platforms ( as well as tracks by way of elevated railway) at Mumbai should have been done atleast 15-20 years back.
This will bring a new lease of life to IR in Mumbai.
Hope CST, Dadar, Kalyan too shall be facilitated with 2 tier platforms.
In Mumbai the 2 tier platform stations shall be more challenging as...
more... the Design & Layout of these platforms have to be synchronised with the plans of the proposed elevated corridors on Harbour,Central, Western lines of Mumbai area. Large number of trains, heavy rush and lack of space are other challenges more specific to Mumbai.