This is a tunnel portal for O Scale Model trains like Lionel or MTH. The exterior dimensions are based off the MTH single and double track tunnel portals. The internal dimensions have been expanded to give more room for trains when the portals are installed near curves.

This is a modular design that can be printed on smaller 3d printers like the Davinci Mini. The internal dimensions of the portals when assembled are:

217mm double track width ~8.5 inches
127mm single track width ~5 inches
146mm height ~5.75 inches

You can print the base pieces and rear connectors without supports. Both upper sections and the keystone need to be printed with supports. I used a .2 resolution with a 15% infill on my Davinci Mini and it turned out really good and took about 20 hours to print.

After printing I assembled the parts with gorilla glue. The connectors snap in enough to hold it well but you will want to glue it. The keystone must be glued in place.

I've updated the keystone_blank.stl file and removed the personalized one. I also made the keystone recessed instead of sticking out. I think it looks better that way.

I don't have a heated bed and have had some pull up on the tops of the top_right and top_left files. Rafts are optional on those if the pull up is too much for you. I can live with it.

Update 3-16-2017:
I've added an alternate top for a double portal that incorporates the expander into the top left and top right parts. This allows you to create a double track portal without any seams visible when using the keystone. I have made these solid. After printing several of these I've found I get less warping if I have it sold and do a honeycomb infill at 10%. Because it was forcing supports in those locations I'm actually using nearly the same amount of filament when printed at .2 resolution and 10% infill. The new files are:

top_left_double_solid.stl
top_right_double_solid.stl

After printing 2 of the singles and one of the doubles I recommend printing the connectors and two bottom pieces at .2 resolution with a 10 infill no supports. The Top portions need a .2 resolution with a 10 infill with supports due to the curve and cavities. For the keystone, once you add text, I recommend a .1 resolution with a 10 infill with supports. It's taking me about 24 hours to print a whole portal.