Buses are available from the Main road outside Santragachi.
If you don't wish for a hassled journey (considering the conditions of Kolkata buses here) , there are 4 EMU trains running from SRC to SHM (3 Dedicated and 1 Mecheda-SHM). Check out departure timings of your train and hop into any of these EMU or else majority of SHM trains (except PNBE duronto) halt at SRC also. Make yourself comfortable.