As someone who regularly visited that route (Ahmedabad to Thane), I advise you take an overnight train (preferably Gujarat Mail) or ADI-BCT superfast. Choose the one that has a halt at Borivali (Duranto doesn't; it has only one stop: Mumbai Central). For morning, double-decker is a good option. This is because Thane is located to the east of Mumbai and commute by trains, whether local or inter-state, does get cumbersome.
Just outside from the Borivali station(East), take bus number 700 which spans the entire run of Ghodbunder highway covering all the major areas in Thane from West to its last stop at Thane Station East.
However, if you decide to stick to trains, get down at any of the major western Mumbai stations (Borivali-Andheri-Dadar-Central) and take a local to Dadar. From there, change the platform to Central line and take a train to Thane.
DO NOT take the passenger as it takes around 23 hours to get there.