We have some retaining walls built using railroad ties. I don't know how
long they've been there, since they were already in place when we moved
in, but they are beginning to look as though it's time to replace them.
If we replace them with new (actually old) railroad ties, how long are
the new ones likely to last? What kind of timber is used for railroad
ties? Anything we can do to make them last longer? -- creosote? The
bottom course is actuually below ground level.

We are thinking seriously about using interlocking concrete blocks
instead. Any thoughts on those?

Perce