i guess bsnl (mumbai circle sim) might work inside metro 
www.freepressjournal.in
So i guess we should be getting 5G now as compared to 4G only so farIn a significant move aimed at improving commuter convenience and digital access, MMRC has jointly awarded the Letter of Acceptance, LoA, to Bharti Airtel Limited, Reliance Jio Infocomm Limited and Vodafone Idea Limited for the deployment, operation and maintenance of telecom infrastructure along Mumbai Metro Line 3.