Generaly if there is Megablock on Mulund - Matunga - Mulund Fast Line in Mumbai, A Special Fast Local Departs Dadar Pf 5 at 3.40 towards Kalyan with Halts at Thane(4.06) & Diva(4.13) and then runs on slow line upto Kalyan. This special Fast EMU runs for passengers of Dadar - Ratnagiri Passenger when it is cancelled between Diva and Dadar.
This is just observation, please confirm at railway station once before travel.