This is the most effective transport mode for Mumbai people.
From morning around 4.00 am till last train is around 12.48 am from churchgate the service is on. There are uninterrupted services going on. Average every 3 to 5 minutes there is a slow local train available. Still you would find heavy rush in all the compartments. When office hours peak period is on it is very difficult to even board at starting stations. But at off time only ladies compartment are somewhat empty. Whole day no-a-days the rush is on.
The main reason is that is very affordable. Even monthly and quarterly passes are available. Hence it is very very cheap. First class compartments are somewhat easy to board at off-peak hours. But the fares are almost 3-4 times of second class tickets. But if you can afford it then its wise to carry family in first class instead of second class.
But it is definite that the local trains services are faster, quicker and affordable than any another transport service like bus, taxi, rickshaw, etc.
In other transports there are definite chances of traffics. Also it takes lot of time in other transports. Like if from Borivali one goes in bus to reach Andheri, it takes almost 1 hour. Whereas from local train it reaches maximum by 20 mintues.