There is one line serving 18 stations in Bangkok, going from Hua Lamphong in the South (near Chinatown) to Bang Sue in the north. Bangkok MRT subway trains run every 5-7 minutes and connect to BTS at Sukhumvit and Silom stations.
There are many places the Bangkok rail network will not reach. In that case you have to take a bus, walk or a tuk tuk. Even though tuk tuks are more expensive I would often opt for them just for the thrill of it.
We loved getting around Bangkok in a bus. The constant clacking of the conductor’s change tin as he tried to wriggle his way through the sweaty armpits of all of Bangkok squashed into the bus is a memory that will last forever.