From the BBC^» (British Broadcasting Corporation - home page)

The article looks at why ....

* Ticket cost

* "Infrequent" service away from the peaks

* Poor interconnections between services - not an integrated system.  Long walks even at interchange stations, services on each piece of public transport siloed and not linked to others

Some of the other background information looks designed to make a point and is an effect and not a primary reason

* Trains are very quiet by the time they get to the final terminal. Yep - there are examples of  buses and trains the world over which run from / through city centres and the final stop(s) are remote - "Triangle loading".  Falmouth Dock or Severn Beach, anyone!

* Trains are short - 3 or 6 carriages.  Well - if there's plenty of space, that makes sense

* Numbers improved by counting each leg of a connection journey as a separate trip in the stats.  So what - it's not going to out people off as how they are recorded.

The answer is, they all still use the extensive, crowded but cheap network of suburban railways that serve the city. I remember briefly travelling on one (being careful to avoid rush hour!) on a short stay there at the end of my only visit to India almost 40 years ago to the day. Not sure anyone had considered a risk assessment for what I recall were the doors being kept open as we sped along....

This thread prompted me to check it out on Wikipedia. The information there is a bit contradictory,  but it appears to handle between 6 and 7 million passengers a day. It also seems like they replaced most of their trains at about the time the metro opened in 2014, and now run mostly 12 and 15 car trains.