South bound trains from Phnom Penh in Cambodia run on a Friday, Saturday and Sunday (and some public holidays) between Phnom Penh Railway Station and Sihanoukville via Kampot.
The Royal Cambodia Railway facebook page has more information on the rail services operating in Cambodia.
The walk from the Phnom Penh Royal Palace area to the Railway Station on 110 street took me almost thirty minutes (propelling my step count into healthy territory)!
I have purchased a ticket for the 3pm train this coming friday to Kampot at $7. The train will arrive in Kampot between 7 and 8pm suggesting a slow journey with variables!
Down The Road 