I had a similar experience. I had a sdg with a row of invisable locomotives in it. The message I got when I tried to place something there was "Cannot place. There is a vehicle in an invisable layer. However i found a work round. If I coupled another loco to them they all became visable. It usually worked on the 2nd attempt as on the 1st attempt the assisting loco became invisable. There was a gap between 2 class 47,s which should not have been there as they were coupled so I detached both class 47,s and sent them to their doom. Problem solved.
