From Bangkok to Ayutthaya by Train

The fastest train of the day is the 8:30am train, which completes the journey to Ayutthaya in 77 minutes.

First-class tickets are not available for local, short-distance trains, but second-class will get you comfortable seats in an air-conditioned carriage for a reasonable rate. You can check train times online, but the reservation system is only in Thai. You can instead book tickets through an agency like 12go, which handles most of the transportation ticket sales in Asia.

Alternatively, you can just walk into any train station and buy tickets over the counter. Third-class tickets in carriages with no AC (which cost less than a cup of coffee and are the cheapest travel option available between the two cities) don't offer a reservation option, so arrive at the station early if this is your preferred travel choice. An unexpected plus of train travel is that there are vendors on the train offering cold drinks, fresh fruit juice, and packs of pineapple chunks to make your trip a lot better.

The railway station in Ayutthaya is not near the temples. You'll either have to take a ferry or a tuk-tuk (three-wheeled motorcycle taxi) to the ruins. If you don't want to walk the entire complex, you can rent a bike or pay a tuk-tuk driver to take you from one temple to the next–an especially good idea on a very hot day.