UPDATE: now also includes tunnel with a slope

With this model, you can print a tunnel compatible with the GraviTrax Junior and extend your marble run building options. This model can be printed without any support and assembled without glue. 

If the pins are too loose or impossible to put inside (it should require a bit of effort; otherwise, they won't hold well), just resize them a bit - I recommend resizing only in the x and y axes and keeping the z unchanged. 

If you want to enlarge your marble run further, you might also look at my collection of all the basic pieces of GraviTrax Junior. If you want to print new decorations for your Gravitrax Junior, you can also look here and here.