In Visitor 4 one can merge two polylines by selecting the vertices and click "Common Node" -> "Merge" (and every other decent vector program has this feature too). This is not possible in Terrain Builder (Option completly missing).
We also have no option to continue the drawing of a polyline so merging lines is a must have feature.
Try to lay down polylines for roads in a city. If you want to expand a road you have to redo the whole line (maybe more than 50km).
A "Continue Polyline"-feature would also be handy (or even better than the merge option).
Terrain Builder Version 1.31
Thanks for an interesting idea. I have just listed them into my TODO list. For now I can only tell you a little workaround: You can RMB on working polyline and click on "Add Point here"
Merging two polylines works now (1.40) when snapping is enabled but "Continue Polyline" is not possible yet.