Steam enthusiasts have at least four opportunities a year to ride a steam train in Thailand. These day trips take place on 26th March (anniversary of the opening of the first public railway), 12th August (Queen’s birthday), 23rd October (anniversary of the death of King Chulalongkorn) and 5th December (HM The King’s birthday). Sometimes there are additional trips. Each trip has a different destination. The steam trains are usually kept at the workshops near Thonburi station when not being used. I have been there before to take pictures though officially it’s not open to the public.
Tickets are limited and so it is advisable to book as soon as they go on sale. The cost of the roundtrip by steam train is 250 Baht. Alternatively, you can usually book a tour package for 999 Baht per person. You can reserve tickets on the hotline 1690. But you would need to go to the train station to pay and pick up the tickets within a few days. You can do this at either Hua Lamphong station or any station along the route. I’m also told you can book tickets at any station in Thailand. So far there is no way to book this trip online.
The train is due to leave Hua Lamphong station shortly after the national anthem at 8 a.m. There is no reason why you cannot go there early in the morning at say 7:30 a.m. to take pictures first. Alternatively, you can wait at one of the other stations in Bangkok. For example Samsen Station or Bang Sue Station.