By bus From Bangkok to Kanchanaburi:
BKS public buses (line 81) leave from Bangkok's Southern Bus Terminal (Sai Tai Taling Chan which is located far west in the suburb Thonburi. In Kanchanaburi, there are two separate but nearby bus terminals, with 1st class buses departing from an office off Thanon Saengchuto, and 2nd class buses from the larger terminal one block east.
1st class buses should leave Bangkok every 15 minutes from 05:00-22:30, take about 2 hours, and cost 110 Baht, including a bottle of water.
2nd class buses (new route) leave Bangkok every 20 minutes from 03:30-19:00 and take about 2 hours. Cost 95 Baht.
2nd class buses (old route) leave Bangkok every 15-30 minutes from 04:00-18:00 and take about 3 hours.
There are also tourist minibuses directly to/from Khao San Road, departing Kanchanaburi at 13:30 and 18:30.
There are also some buses leaving less frequently from Bangkok's Northern Mo Chit bus terminal (note: not the same as Mo Chit BTS station, and not within walking distance of it, although a standard 50 Baht motorbike ride is available. It's often called "Mo Chit 2"). Here are the times I could find at the station:
First-class bus with toilet (3 hours, 122 Baht): 06:00, 11:00, 14:30
Second class bus with no toilet inside: 05:00, 07:00, 09:30, 12:30, 17:00
Bus rides may be variable or cancelled (for example, with 14:30 being last of the day.) BUT there are vans available at the bus station leaving even when you're told there's no way to get there by bus! It may pay to talk to the information desk for this. Price Feb '11 was around 120 Baht, about 2 hrs.
Tips: best to buy ticket at least one day in advance. As many other Thais outside Bangkok, the company staff cannot understand & speak English well, especially over the phone. If there is any problem communicating to the staff, consider asking your hotel/hostel staff to speak to them in Thai over the phone, i.e. asking for the bus station address and have the hotel staff write it down in Thai - taxi/tuk tuk driver could find the place easier that way.
By train
Trains leave Bangkok's Thonburi Train Station at 07:50 and arrive at Kanchanaburi at 10:25, also at 13:55 and arriving at 16:24. You may be interested in buying a ticket all the way to the River Kwai Bridge, since these two trains are the only ones which cross the bridge each day. Since December 2005, the fare is 100 Baht for foreigners.
Be warned that reaching Thonburi Station from Khao San Road is harder than it looks; tuk-tuk drivers will try to charge you outrageous rates, and walking involves crossing two bridges and looping back a way. The best way is probably to take the passenger boat from Phra Arthit Pier and connect to a cross-river ferry that reaches the Thonburi Railway pier, then walk or take the open minibus from there. The other way to reach Thonburi (Bangkok Noi) Station is walk little bit south from Thammasat university (next to Sanam Luang), take river ferry (3THB) from Wat Mahathat to Wanglong, on the other side of river walk west until you reach main north-south road, here walk north until you reach big bridge where you will go under it and then just walk west to train station, all this from Khao San Road to train station takes only about 30-40 min's easy walk + river ferry. You can also walk a bit away from Khao San Road and find a metered taxi that will not rip you off. The fare should be about 70-80 Baht from Khao San on the meter. If you miss the train taxi from Thonburi train station to Sai Tai Mai bus terminal for buses to Kanchanaburi (110 THB) costs 80 Baht.
