Railroad ties would seem to be an easy solution, but in fact they need quite a bit of preparation, including a gravel base and "deadmen" every few feet. A deadman is a piece of railroad tie attached to the wall that is perpendicular to the others, going back into the soil behind the wall to act as an anchor so the wall won't sag forward. Retaining walls are subject to hydrostatic pressure when water builds up behind them, so drains are often required to bleed that off. No matter what you do, the wall will eventually rot and need to be replaced. This article explains more:
How to Install Railroad Tie Walls | Home Guides | SF Gate
The easiest way to build a retaining wall is to use concrete blocks made for that purpose, which come in a few different colors and interlock with a lip at the back. They're certainly easier to move around than railroad ties. Once you put down a gravel base and level it, it's just a matter of stacking them to the desired height. Hydrostatic pressure isn't a problem with low walls because the water seeps through the joints. Hope this helps.
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