The perpendicular connectors that keep the rails in the railroad track equally spaced. They used to be made of treated lumber, but mostly are made of concrete nowadays.

ASCII Art diagram of a segment of railroad track:

        rails
       /
      /\
     /  \
    /    \         
                   
  ||      ||
.-||------||-.
`-||------||-' --.--.- railroad ties
  ||      ||    /  /
.-||------||-. /  /
`-||------||-'   /
  ||      ||    /
.-||------||-. /
`-||------||-'
  ||      ||

The type of wood that was used to make wooden railroad ties include oaks, gums, Douglas fir, mixed hardwoods, hemlock, southern pine, and mixed softwoods.