When using reclaimed sleepers be careful of the creosote, tar and other nasties in them. They can seep into the soil and kill any plants nearby. Apart from that they are a lovely material to use.
(INSTALLATION OF SLEEPERS)
By the morning your posts would of set solid, So place your sleeper on its side against the front of the post, ensure its a snug fit, at each end make sure the adjoining sleeper fits flush, plane the edge if needed.
One your sleepers are in place use a CORNER BRACEor L BRACKET (3" or 4") fix the sleeper to the post, the bracket should sit on the top half on the sleeper and half on the post. on the first row there should be one on the centre post and one on the edge of the end posts. screw the brackets on using decent quality screws (4") galvanised if possible.
Fix another couple of brackets behind the wall towards the bottom, this will ensure the bottom does not kick out under pressure.
fix the sleepers together with galvanised 4" nails on each edge of the sleep on a angle into the next sleeper this should be done top and bottom
To install the next layer of sleepers you will need to do a sleeper cut, this is around 4.3 feet and this will connect from the first post to the middle post, this will then create a bond in the wall and the posts should all meet like the first row installed first.
As this is the finishing level you will need to fix 2 brackets to the rear of this sleeper so they cannot be seen, add plenty of nails, avoid splitting the sleeper.
You now have a finished sleeper wall of 20" ....to make it 30" just make the posts bigger, and repeat the process.
Have a friend who's a geotechnical engineer. He always bangs on about the fact that a retaining structure needs mass, so for example those massive nets full of rocks stacked; sleepers on end as JLD says, the 1/2 in the ground means that the amount of force exerted on the upstand can never (in normal use!) overcome the mass against the buried half.
Charles - If your talking about strength and longevity I like how Steve Snedecker(? Member on here, U.S.) said that he did sleeper retaining walls.
Laid flat pinned together with 20mm steel bars with a 4'header every 8' or so into the retained earth. This header in turn has a 4' long section of sleeper bolted to it.
The strength done this way would be colossal. The whole point is that this retaining structure is being held by the weight/mass of the retained earth - Very clever :)
This is how I will construct in future. The fixings used should outlast the sleepers....
Must admit I don't like the look/sound of the angle plates, or screws! When they're rusted away the posts will do jack!