I was laying down some train track with the latest version 0.6.7 today and the drones would go out, lay down some track and then seemed to just get stuck on a piece of track that was already down. They would just circle the existing track. I couldn't manually pick them up and they would not return on their own. My inventory was not full. I deleted the track they were standing on and then they would return. This happened several times with them getting stuck on existing track in different places. The track they were stuck on was a curve, not sure if that is important but thought I would mention it. Not sure if they had track in their inventory but they did not seem to realize that the track they were on had already been placed. Reverted to the previous version with no problems. Really like the mod.
