Sunday, November 19, 2017


Adding tunnels to my railroad involves considerable preparation prior to closing the area in with terrain.  The previous post described building masters for tunnel portals and half of a concrete liner.  It also described making the silicone molds using the masters.  The post can be viewed at:

The tunnel project has now moved into a production casting phase.  I am casting the portals and tunnel liners in hydrocal, a US Gypsum product.  It has been a couple of decades since I last did any casting work for a model railroad.  I needed to relearn a couple of lessons in casting such as how fast hydrocal sets up and how to get it into the mold detail.  My first casting attempt used too much mold release.  It pooled in the mold, obscuring the detail.

First casting of two portals and a half tunnel liner.  Note the yellowish fluid on top of the half tunnel liner casting on the right (tall mold).  This is some of the excess cooking spray used as a mold release.  The resulting castings had very little of the form plank detail of the masters and molds.

Subsequent casting attempts have been far more successful.  I still suffer a little bit of trapped air bubbles, now mostly due to how fast the hydrocal is setting up.  Fortunately, the small bubble impressions are very minor flaws and easily corrected with some spackle. 

Tunnel portal castings.  The form plank detail built into the masters shows with this lighting.  The “1926” tunnel date centered above the portals came through, as well.

As I go into production on these tunnel parts, I am reminded that it takes a while for hydrocal to cure.  There is a lot of water contained in the castings.  I will wait for the castings to cure more before painting.  Meanwhile, I will do some shaping and finishing.  As seen in the portal photo above, I have a bit of mold overflow (like “flash” in a two-part mold) to trim off the back of the portals.

Half tunnel liner mold. The scribed siding lines and a bit of form plank detail is visible.  This mold has been sprayed lightly with mold release.

Cast tunnel liners.  The pair on the left has been positioned the way they will be used.  I need to trim the mating surface on the tunnel top centerline.  The liner half on the right shows the modest detail I built into the master.  Tunnel liner detail quickly gets lost in the gloom inside the tunnel.  Still, the liners will mate well with the more detailed portal castings.

I am now in production on these important tunnel parts.  With ten full tunnels, each requiring a portal and a concrete liner at each end, I have a lot of casting to do.  Consider this a “price” of mountain railroading. 

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