There never has been a railway line to Siem reap, and there are no plans to build one in the future, however the old line from the Thai border at poi pet to the capital phnom penh via battambang is at the time of writing being reconstructed.
The only possible routes of getting to Siem reap are by bus and by river boat.
Siem Reap is easily reached by bus from anywhere in Cambodia, but if you’re coming from somewhere far away like sihanoukville it might require a change of bus in Phnom Penh. The capital of Cambodia doesn’t yet have a central bus station, instead each of the several national bus companies has its own terminal, which are scattered all over the city. If you are unfamiliar withthe city, the best way to book a ticket is to go into a travel agent, and ask for a ticket to Siem reap. Travel agents are all over Phnom Penh especially areas popular with tourists like the riverside so they should be very easy to find, possibly hotel reception also sell bus tickets. You’ll pay a small commission for the ticket, but this will include a pickup from outside your hotel or guest house, so is far easier than trying to find the bus station. Bus companies running to siem reap include Paramount Angkor, Rith Mony, Capital tours and Mekong Express. The latter is most expensive but is generally regarded as having the best busses and fastest journey time. There are also now some Transit sized mini busses that are even faster.
The quality of the roads has improved immensely in Cambodia, now all the major highways are sealed and for the most part in pretty good condition barring the occasional pothole, meaning times of around five hours for the trip from Phnom Penh.
There are boats which make the daily trip to Siem Reap from both Battambang and Phnom Penh. In both cases they are much slower and far more expensive than the competing bus route, so they are aimed only at tourists. The Phnom Penh one takes a particularly long time, sometimes more than ten hours, and as its mostly travelling on the tonle sap lake doesn’t really offer amazing scenery, so I would say give this one a miss. The boat to and from Battambang is more interesting, and stops for a meal at one of the floating villages that dot the great lake. Be aware that travelling from Battambang to Siem Reap is faster because the boat is travelling downstream, so a good option is to take the bus to Battambang, stay a night and then return by boat.