I am building the Nickel Plate Road in 4x10 using Lionel Fastrack.
Here are a couple of pics of construction so far: mainlines up and running well:
I have a track planning article with discussion submitted to OGR. I plan a follow-on article on the construction of the layout.
Here is the layout section design:
And here is a short clip to demonstrate the clearances: about 4.25" centers. Its tight, but everything works, almost but not quite touching. The locos in the clip are: on the outside main, the SCALE SD60M Norfolk Southern LC+2.0 pulling Menard's double-stacks with the camera caboose at the end (fun!). On the inside the BNSF Tier 4 LionChief pulling an assorted BNSF train. I have run these trains for about 2 hours total, with no problems.
The above clip is before a slight re-alignment, the re-aligned plan is shown in the track plan diagram. So I will record another clip after I adjust the track on the layout. The adjustment adds about 3/16" to the outside mainline width, to add just a bit more clearance (for comfort) .