We took the slow strain from Thonburi station in Bangkok to Nakhon Prathom. I have never come to this station before and a little hard to find for me, a bit outside Bangkok in another side of the river from central Bangkok. We took the slow train number 257, leave at 07.45 am and arrived at 08.50 am, spent 10 B for train ticket. There are some more slow trains leaving Bangkok in the morning and passing by Nakhon Prathom.
I took a bus from Southern Bus Terminal in Bangkok just about 1 hour for a trip. The bus stop 's in city center ,so you can enjoy walking around from the bus stop to some interesting place like Pra Pathom Chedi.