I have just stayed for one night at @ Hualamphong Guest House / Hostel and thought it worth mentioning on Travelfish. In terms of location and proximity to Hualamphong it would be hard to do any better. The guest house is essentially opposite the station if you exit via the front (where the KFC is). To get to the hostel you can go into the underground and cross the street, coming up at Exit 4. If you are struggling, it is also close to a large and raised branch of Siam Bank (red frontage). All in all, 3 minutes from the station.
They have a a choice of rooms including dorm beds (6 mixed) and double rooms. A dorm is 300 baht off the street. I stayed in the dorm room so can only comment on those. The beds are wide and comfortable, with very clean sheets. The rooms themselves are kept clean and have en suite shared bath room (2 shower, 2 toilet). The dorms have an air conditioning system which you have complete control over and the room electricity is controlled by slotting your key card into a switch in the wall. Each room has a large locker which can fit your large pack and day bag with a little room left over. They aren't the most secure (just one of those tall metal lockers with the lock in the middle) but fine for shoving all your things in. Even though I was on the third floor and in the 'other building' (they have a building behind the main one which is joined together with a fire exit) I still managed to get wireless (weak but workable) in my room.
I have read on hostelworld that the staff here are variable - from very helpful and pleasant to not so much... well, people are people, not everyone is sweetness and nice all the time. Overall, the staff provided good information whenever I asked - both with a smile and without. There is 24 hour check in and the door from the downstairs up to the rooms is electronically locked after a certain time.
The guest house has a communal room on the first floor that centers around a large flat screen TV. I believe it just has come kind of Thai TV box in front of it. I have never seen anyone in here so it is a good place to go for peace and quiet (or yoga :)). This room has air con which you can turn on or a fan. The ground floor of the hostel comprises of 3 pay-for computers (10 baht for 15 minutes) as well as a small Cafe serving western and Thai dishes. Guests can sit in the cafe (it opens out onto the reception area) and use the wireless free of charge. You can also pay for wireless as a non-guest (40 baht). The food is reasonably priced when compared to any other tourist type of place - at least in my experience. I am not sure how the drinks relate to other places but the cost of a bottle of Leo is 53 baht and a pot of Thai tea with sweet milk on the side is 55 baht (about 3 or 4 cups).
In terms of transportation, it is obviously well situated for train travel out of Bangkok. In addition, you can catch bus 73, 507, 529 or 542 to the Siam / MBK center area. Just walk to the opposite side of the hostel and you cannot miss the stop. The 73 is an air-con bus, costs 11 or 12 baht and takes about 15-20 minutes to get to MBK. To return to the hostel, simply catch the same bus from the opposite side of the road at the same place. You can also take bus 53 to Khao San Road, although I have not done this. Finally, the subway station is literally on your door step.
You are around 10-15 minutes walk from Chinatown which is a big plus for evening food when the place explodes with street stalls. There is also a Tesco Lotus and the usual array of 7/11's dotted around the place. There is a massive maze of an indoor-ish market that I got lost in too, selling everything.
With regard to traveling solo (which I am) there seem to be a steady stream of people staying here, although there were only 3 of us in my dorm and all using the places as a departure point to other places. There do seem to be a number of couples around - in the traditional sense and just pairs of friends. A good mix of ages and nationalities too.
Overall, I like the area a lot and would definitely recommend staying here if you are in transit or just want to try somewhere new for a short time. The guest house is nice but quiet (negative or positive) and not so communal. It is a good, clean place to rest your head with many of the facilities you would need when passing through.