The entrances for Domestic airport mumbai and International Airport mumbai are different, but runway is same. For Domestic airport there are two terminals 1A and 1B. To reach domestic airport nearest local railway station is Ville Parle. Get down there and come outside to Ville parle (east).From there you can take an auto for a max distance of 2 KM. Or can have number of buses plying from ville parle east station to airport.
For International Airport in Mumbai the nearest local railway station is Andheri. You can get down there and come outside to Andheri(East) and from there you can take Auto max of 3 KM. Here i don’t have an idea about the terminals but all at same side only.