At the present time it is not possible to purchase train tickets for Cambodia over the internet. You can only buy Cambodia train tickets at a train station and only up to 1 day in advance. In the meantime you can, however, book bus tickets in advance and over the internet by clicking on the BOOK button.
Click on the links in red for travel times, journey information and ticket booking:
From Phnom Penh:
From Siem Reap: