Yes, creosote does leach out of the ties and into the soil, but worn-out ties are generally not a problem, because most of their creosote has already leached away. Whether plants take up the creosote has not been settled. However, because creosote is toxic, new ties can cause growth problems for plants that are sensitive to it.
You can line your beds with plastic to prevent contact between the soil and wood, if you like. If the wood is oozing black creosote or has an odor, it shouldn't be used. Gases released from creosote are also harmful in a closed space, so railroad ties should not be used in a greenhouse or indoors.
Many other materials can be used for constructing a raised bed, so there is no reason to give up the idea. Because chemically pressure-treated wood is expected to last up to 40 years, most folks turn to it first when building a raised bed. If you are concerned about safety, however, be sure to research your options. Although all wood preservatives have guidelines for safe use, some of them, such as ACQ, are thought to be safer than others.
To avoid preservatives completely, choose from among several woods that are naturally rot-resistant, such as heartwood-grade redwood, knotty red cedar, cypress, catalpa, juniper, or Osage orange. Construction-grade heartwood can last so long that you'll probably want to redesign your bed before it falls apart. Availability often depends on your locale. The plastics industry has other options, including recycled plastic and plastic mixed with sawdust. They look and handle similar to wood, will last for years outdoors, and don't leach any chemicals.