The State Railway of Thailand organises steam train excursions six times a year on important public holidays. These go to three different destinations. The trips to Chachoengsao go on12th August and 5th December. You can buy tickets online or at your local train station up to 30 days in advance. Even if you don’t buy tickets to join these trips, it doesn’t mean that you cannot go to the station to take photos. This page contains my notes on how to take photos of the steam locomotive on the days that it goes to Chachoengsao.
The best place to start is at Bangkok Railway Station, otherwise known as Hua Lamphong (see map). Unlike in other countries, you don’t need a train ticket or even a platform ticket to go and take photos. The train leaves at 8:10 a.m. I suggest that you arrive by 7:00 a.m. at the latest so you will get a good hour to take photos. If you come in your own car, you can park at the station car park for 20 baht per hour. For a full day, ten hours or more, it costs 300 baht. There is also an MRT station called Hua Lamphong which is a convenient way to arrive.