The accident took place at 6:36 am.
No passenger is reported to be injured in the incident so far, Sunil Udasi, Chief PRO of the Central Railway, said.
Udasi said the derailment was caused due to a landslide.
"The cause (of derailment) is sudden landslide due to heavy rains. But graciously enough, the driver did an excellent job of applying emergency breaks," he said.
However, another railway official said the exact cause of the derailment was not yet clear.
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The train had a total of 18 coaches.
Helpline numbers have been set up at CSMT, Thane, Kalyan, Dadar and Nagpur stations to help passengers.
A PTI correspondent, Justin Rao, who was on-board the train said, "At least six coaches along with the engine slipped off the tracks".
Rao, who was travelling in the A-2 coach that derailed, said the accident happened when most of the passengers were sleeping.
"Few people were stuck in the washrooms and were rescued by some passengers by breaking glasses of windows," Rao said.
Rao said, "Though relief and rescue came a bit late, everything was fine once the Central Railway team arrived."
A team of doctors as well as senior officers are at the spot.
Proper arrangements have been made to transport stranded passengers from the site to CSMT or other destinations by buses and taxis.
Traffic operations in the affected section have been stopped and few trains have been diverted and short- terminated, official said.
"Sectional officials and railway workers have arrived at the spot and the entire team has swung into action to rescue stranded passengers and restore traffic in the affected section," Udasi said.
"Our rescue team along with the Accident Relief Train along with engineering staff has reached the spot," he said.
This is the fourth train derailment in the country in a a span of 10 days.
About 100 passengers were wounded when 10 coaches of the Kaifiyat Express train derailed in UP's Auraiya district after crashing into a dumper which strayed on to the tracks.
On August 25, six coaches of the Andheri-bound local train derailed in Mumbai, injuring six of its passengers.