This route works best as Bangkok → Ayutthaya → Bangkok or Thonburi → Kanchanaburi. Ayutthaya and Kanchanaburi sit on different rail corridors, so do not plan on a through train between them.
Give Bangkok at least two nights, Ayutthaya one full day or one night, and Kanchanaburi two nights if you want more than the town itself. With fewer than five days, choose Ayutthaya or Kanchanaburi rather than spending the trip changing bases.
Use the State Railway of Thailand booking service to check a Bangkok–Ayutthaya train for your actual date. Search the station names, not a generic Bangkok transport map. Road transport can be quicker from some parts of Bangkok, but the departure point and operator need a current check.
If you only want the major ruins, Ayutthaya can remain a day trip. Stay overnight when an early start, riverside evening or museum visit matters more than returning to Bangkok the same day.
No direct rail connection was verified. The dependable planning assumption is to return toward Bangkok, then use the separate western service from Thonburi to Kanchanaburi. A direct road transfer may reduce backtracking, but confirm the current operator, pickup point, luggage rules and destination before booking.
The State Railway of Thailand's western-line visitor page identifies Thonburi as the Bangkok terminus for ordinary services through Kanchanaburi toward Nam Tok. Published times can change, so use the live booking service or SRT hotline 1690 for the travel date.
Drop Kanchanaburi if returning through Bangkok creates too much transfer time. Drop Ayutthaya if your priority is the western railway, waterfalls and a slower Kanchanaburi stay. Heat, rain and the distance from Kanchanaburi town to outlying attractions can turn a neat map into a rushed trip.