Thailand forum

Yes, it would be the same train to Bangkok.

I wouldn't bother buying a return ticket unless you're completely dedicated to certain travel dates, but you may want to get one in advance just so you have your sleeping berth reserved before you leave KPG. Also, if you are going to be traveling during a holiday or weekend, it may be a very good idea to get your ticket in advance.

As for the combo ticket, it may be convenient, but it's very easy to get there on your own. You could get off the train in Nakhon Pathom then catch the train to Kanchanaburi, or go by bus if you have to wait a long time for the train. I think the train to K'buri stops at / departs from Nakhon Pathom at around 9:00 am. So if you get your timing right, it could be seamless.

If you tell the ticket seller that you want to go to K'buri from Surat via Nakhon Pathom, they'll get the right train for you and sell you a ticket that will take you straight through, though you won't have any trouble getting onto the train in Nakhon Pathom if you show up without a ticket.

One more thing. When you go to Hualomphong Station in Bangkok, there is a little booth near the doors on the side of the station that serves as an info stand. There are usually eager, young, English-speaking Thais hollering "Where are you going?" and stuff like that. Most people just walk past them, thinking they are trying to sell something, but they are actually very helpful and have printed versions of timetables for all of the lines ... North, South, East, Northeast. Go and tell them what you want to do. They'll give you a printed timetable and will circle the train(s) you need to catch. You can take this to the ticket booth and then keep it for future reference.