RE: Tunnel & Portal Clearance
Hi Matt,
Good question, and I'm looking forward to seeing what you accomplish in the way of a stone portal, as I have toyed with the same possibility myself. To date I only have one tunnel (on a straight) which presently has wooden portals. I model 1:20.3, and run Shays, Moguls, a Connie, and more recently a K-27. Here's what I have done: Tunnel portals (remember, on a straight) 7 3/4"W X 9 3/4"H. Comfortable on width without being overly wide. 9 3/4"H, which is probably just a little generous. Despite the 'K' the tallest piece of rolling stock I own is an AMS caboose which just clears a bridge which is 9" above the rail head. The 'K' is a little less than this.
Like you, I have sort of standardized with 10' dia. corners. I do however have a helix at 8 1/2', and other hand bent corners at 11' -13' dia.. The 'K' while being about as fat as a Connie, has a longer wheelbase, and hence has a greater swing on the pilot. My tunnel is not on a curve, but the areas of my R.R. which have curves often have rocks either side. When the 'K' arrived, out came the angle grinder! I found the 'K' needed about 3 1/4" clear from the outside of the railhead on the outside of the corner in order to have clearance. This has not as yet taken into account expansion of rails in summer, and the resulting relocation of track, so use this figure as a guide, rather than gospel.
I love the old narrow gauge look, of tight clearances, and narrow paths. Incumbent upon this thinking is the requirement to be responsible for variances in tight tolerances due to things like track expansion, which will literally 'Move' your track around. Lastly, curved tunnels can be done, but can also be a recipe for disaster if they are not big enough, or you don't provide access to remove a derailed train. They can be a real pain in the butt to re-engineer! Good luck with your portals, and make sure that you post some pictures! Inquiring minds need to know!
Regards,