LibreCAD - An open source 2D CAD application for Windows, Apple and Linux.
SCARM - Simple Computer Aided Railway Modeller
FreeCAD - An open-source parametric 3D modeler
RailModeller - RailModeller is a new application for creating model railroad and slot car layouts.
SketchUp - 3D for Everyone
TrainCad - Freeware Microsoft Windows program for designing model railroad layouts using standard tracks.