If you are flexible with your travel dates, always consider that you can buy extremely cheap train tickets if you avoid travelling in the weekends and if you choose to travel during the low season.
If you don't feel like spending many hours in a train, you can consider travelling by plane. Especially if you have the possibility to book in advance, flying to Bangkok can be definitely cheap.
Which stations do the trains to Bangkok leave from?
You can take the train to Bangkok at Chiang Mai, which arrives at Sam Sen or Bangkok.
What time do the trains to Bangkok leave and how long is the journey?
On average, it takes 13 hours and 1 minute to get to Bangkok by train, but you will need 10 hours and 35 minutes with the fastest trains. The first one is available at 06:11, whereas the last one leaves Chiang Mai at 18:11.