Priya Prakash Sawant (30) boarded the general compartment of a local train at Ambernath station to go to Mumbai, Thane station manager N Mahidhar said.
At Ulhasnagar, a commuter carrying a bag boarded the same compartment as the woman, along with other fellow commuters.
When the train arrived at Dombivili station, the man alighted leaving the bag behind, he said.
Priya, a resident of Lower Parel in central Mumbai, and another passenger Ganesh Patil, examined the bag and found Rs one lakh cash in it, but there were no documents to establish the identity of the person.
Mahidhar, while appreciating the alertness and honesty of the two commuters, said the Central Railway authorities would try to ascertain the identity of the person who left the bag, adding that the cash currency contained the labels of SBI.