DMRC is probably unique in the country in having a rigid catenary. On IR's main line tracks, tunnels and bridges are constructed in such a way as to accommodate all kinds of trains (freight and passengers) and are also constructed to be suitable for electrification. So IR's main line routes use only the normal kind of electrification.
Metro lines however, pass through congested suburban areas and unlike main line routes, it is not always possible to build bigger or wider tunnels as it is more restricted than the main line routes.
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more... Also, it's not a simple matter of switching between both types of electrification. Locos and their pantographs have to be redesigned to be able to run with the rigid catenary systems.