That would be regular face extrusion that is one of the most common modelling techniques (box modelling) besides sculpting, can be also a stage before sculpting.
I can only give some rough description, you might consider opening a job offer if you need a full implementation. Asides of all the basic stuff like maintaining the vertices, triangles edges etc, you basically detach the selected face when extruding and add triangle quads to the edges it has been detached from, extrusion can be just along the pulling direction or scale accordingly to the face normals.
Regarding paths or railroads it’s a bit more complicated as you’re
- working with a repeating geometry rather than shape
- intersections need to be handled if you don’t want just overlaps
- without a proper LOD system it won’t scale well
Here’s some post about such node networked based paths, it’s just road decals in this case but i use the same for on-the-fly geometry parts like railroads or fences in my game, and in distance railroads are just flat decals again like roads Wall building in a level-editor
Otherwise memory grows quickly in large tracks or becomes expensive to construct or render.