You can access this dialog from the following:

Ribbon: OpenRail Modeling > Miscellaneous

Creates sleepers. Sleepers are created for the track and turnouts. In the turnout locations, long sleeper placement is applied based on the turnout feature definition. Cant is also applied while placing the sleepers.

Sleeper tool is used in 2D model similar to other tools. But sleepers are only placed into the 3D model and have no representation in the 2D model.