Try a product called "Quick Set". It is a concrete product which sets very quickly. You can get it with or without gravel and it sets up very quickly. I would think you could use the same oil thinned with mineral spirits as the other gentleman mentioned as a mold breaker. Also, if you want a detailed mold, such as rock, you could cast a straight flat blank of plaster and carve your rock portal from it and then pour molding rubber over it to make a rubber mold. All of this material can be found at a good crafts store along with instruction booklets on how this type of mold making is done. I've also seen the instructions in many back issues of various model train magazines. I've only been into model trains for about a year and a half but I discovered this method early on and am planning a stone railroad station in the near future. I will use this method to model the stone walls of my station.
Oh yes the Quick Set can be purchased at any hardware store which sells Sack-crete or this type of concrete products.
Bill Wagner
August 12, 1998
I've had good luck with commercial "concrete mix" which contains gravel up to about 1/4 inch size, poured in wooden forms that are just tacked together so that I can take them apart to remove the casting. Make the internal form that forms the opening in the portal in at least 3 pieces so that they can give a little, as they will expand a little when they absorb moisture from the concrete. I use motor oil thinned with mineral spirits (paint thinner) applied liberally with a brush as a release agent. It works reasonably well, particularly if I scrape the forms clean and reapply the oil between uses.
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Hints on how to cast tunnel portals.