Pete, at first I used standard 2X12's the first year. I only built two as I still wanted to experiment with it. I also had the "Normal" garden next to it. After the 1st 2 seasons, I went 100% raised beds, and haven't looked back.
I now have them built out of landscape timbers, not railroad ties. I use the square/oblong shaped timbers that you might also see used for fence posts?
I built them in a "Log Cabin" fashion by overlaping the timbers. I then used the special "Landscape Timber Screws" to hold them in place, and then I drilled 1/2" holes using a bit extension every so often and drove 2' long 1/2" re-barb thru those holes thru the timbers and into the ground. I have 2 rods at each end of each bed, then there are 5-6 of them along each side, I think? The beds vary in width of 3-4' but they are 16' long. Height is roughly 12-14" high as I used 4-5 layers of the timbers.
Prior to securing the bebs to the ground, I layed 2 layers of the weed barrier under them. The soil here is hard clay, so I go to a local place that sells mulch and such, and they offer what they call a "Garden Soil Mix". It is pretty much good soil, sand and compost mixed in thirds.
There was some work to put it all together, get the soil, etc. BUT it was WORTH IT! I now grow everything I used to grow the standard way including corn, potatoes, carrots, etc.
The best thing is that by using the raised beds, the soil doesn't get compacted. I can take my hand and dig into the bed and scoop out the dirt easily, it so easy to work. I pull out all my plants & stalks at the end of the year, and mix in some fresh compost and turn the soil using a pitch fork. It also takes less watering, the soil is also warmer in the early spring and late fall due to it being elevated, so you can plant earlier in the Spring as well as put things in later in the year as well.
Did I also mention that there are less WEEDS to deal with? This allows me to plant my items very close to each other. I stagger my corn in a "Z" fashion in the beds as well as my tomatoes/peppers. Another good thing about the beds is that I have attached posts to the sides of my beds using deck screws, and I then built stringers across the bed for my vining plants to grow on. Never liked the wire cage things for staking plants and such.