The train to Kanchanaburi goes from Thonburi Station in Bangkok, that’s not the main train station, it’s a tiny one. It’s not far from the Khao San Rd area. We payed 150 Baht from the Khao San temple to Thonburi station, you could probably find a driver who would charge you less or, ideally, use his meter. We beat this guy down from 200 and then lost the will to fight, he was the sweetest guy, he made the kids little origami flowers while we were stuck in traffic jams. Taking a river boat most of the way may be a cheaper option, the station isn’t far from the jetty. The train costs 100 Baht each. We had to pay full price for both of the children ( 6 and 9) The Bangkok to Kanchanaburi train leaves at 7.50 am or 1.55 pm. Just turn up, no need to book in advance.