In order to move forward on my layout,I found several portals of various sizes were needed. After searching the internet, magazine ads, and local hobby shops, I was disappointed in the available choices. For the few I did like, the sizes were all wrong for me. I have my tracks set rather far apart to accommodate the larger engines and passenger trains I run. So it seemed the only way to move forward was to reach into my bag of tricks again and make my own. First I went to the "National Model Railroad Association" website and found a section on templates. A template is nothing more than a shape that's used to setup the clearances needed for the opening in the portal. Here's a link to the site: http://www.nmra.org/standards/consist.html
I drew up what I wanted based mostly on intuition and visual appeal. The end result is a combination of portal styles I've seen and liked .I wanted to simulate concrete so I choose to cast them from plaster. I wanted the portals to be smooth so I built a form from scrap lexan and wood. When making your mold try to bevel all the edges of the pieces used to make the different shapes .It will aide in the removal of the casting. Here's a couple of pictures of my mold:
I cast the portal using Hydrocal Plaster from http://www.plaster.com/HYDROCAL.html. I dismantled the frame and very gingerly separated the casting from the form. I had a little spot on the mold that allowed the plaster to flow under it and that resulted in a chip . I fixed the gap on the next one. I also rubbed the surface of the mold with plain old pure silicon caulk and let it dry overnight. The caulk worked great and I've had no more chipping issues.
I made it bit tall to accommodate various locations on my layout but it can be trimmed down with a small hand saw. An important note here: Avoid grinding on Hydrocal with abrasives things like a Dremmel with a diamond cutting disk, belt sander, etc. and creating dust. Hydrocal, like most plasters and masonry products, contain crystalline silica. It's NOT in its finest (breathable) form in the plaster mix or casting and the MSDS (manufacturers safety and data sheet) will support this, but grinding it can free these particles into the air. This is a BIG DEAL!! Crystalline silica can cause CANCER!! So please, use a respirator or just don't cut/grind it with power tools. I use a small hand saw.
Here a couple more I made using this method:
The possibilities are endless. I'm rather pleased with the results. I could use some ideas on coloring it. I was thinking along the lines of a scenic express wash or something. I have several places that require rather large portals including one for a three rail tunnel. This should allow me to build any size I need.