The 73-year-old actor took the Electric Multiple Unit train from VT (Victoria Terminus) to Bhandup and sang his hit songs during the journey.
"Local from VT (Victoria Terminus) to Bhandup to support Saurabh, who sings in locals for charity to cancer... Sang with him," Bachchan tweeted.
Saurabh plays a guitar and sings on local trains and distribute the meagre amount he gets from co-passengers to the less fortunate cancer patients and their families in the city.
"... His story was heart breaking... I felt I would want to sit by him and travel the distance he does, do what he does, in a show of support and spirit for his incredible thinking.
"To hope that many others would feel the same way as we did when we heard his story... And perhaps that it may sufficiently light a small flame of hope for many other deeds that need to be done.