The drivable will now be visible. Additionally, you may want to allow collision detection for the drivable. This requires setting the collisionheight and collisionradius properties. Follows the terrain, will not turn without a velocity. Will always hang vertical.
Length is influenced by the player.
For example, you might mount a rocket launcher on a mountpoint on a vehicle. Most cranes and all trains move along tracks. You can layout the tracks, then create the path, using DrivablePathNode actors. Knows how to fire projectiles. You can put multiple gunbarrels on a drivable.