Bit of a brainstorm last night, as you do, about four lane tracks, have done a bit of digging and it appears the easy way is to take a second power base and fit it backwards to give a four lane start finish line. Then reverse the power feed to it so all the cars still move in the correct direction.
Will be giving this a try, have ordered a few DPDT switches and a little box to stick it in and will be torturing the power pack when they arrive, just need to find my soldering iron
since Radius Three will fit on a 4' wide table the idea of a four lane track with a mix of R3/2 and R2/1 curves should fit fine in a nice figure of eight with a bridge
experimentation ahead