Dear @Alok
You can register your complaint about lost items or properties in a Mumbai local train through the RailMadad Portal (railmadad.indianrailways.gov.in) or the Western Railway’s grievance portal (wr.indianrailways.gov.in). Be sure to include details like the date and time of the incident, the train route, a description of the lost item, and the location where you lost it.
For more help, contact the toll-free number 139 or the station master/railway police/grievance redressal helpdesk at the respective railway station of the Mumbai Local Train.
To know more, read this: Where can you file a complaint about Mumbai Local Train?