Face in sliced file too much

I have two idintical drawings of a switch which I deigned. Both I exported to a STL file, then I loaded each of them in CURA for slicing. One comes out correctly the other has one face too much in between two sleepers, though both drawings are identical. Below is the screen shot of both switches and the screen shots of the sliced files. Can any body tell what is wrong?

Perhaps it has something to do with the sleeper faces being reversed. You shouldn’t have any blue faces exposed.

Did you try the model i drew? It doesn’t have any reversed faces and it reported as solid by Sketchup.

I assume the last screen-grab is in Monochrome mode.
There are several blue-gray faces indicating that they are reversed.
A solid should only display the front faces.
You should never be able to see back-sides of faces.
If you can they are wrongly oriented and need reversing [context-menu] or some faces are missing and you are looking inside - either way it’s wrong.

Use Monochrome mode to view your work as you go, and ensure that no back-faces are showing [use a style with a distinctive back-material].

Of course an object can also be rendered non-solid by having edges which don’t have exactly two faces associated with them - i.e. faceless edges, edges with only one face [flaps, shelves, holes], and of course internal partition faces where some edges will have three or more faces…

What is meant by reversed face ? I see these faces are blue but I do not what is the caurse.

Yes I sliced you drawing of which the sliced version is below. I have two reasons why I redrew myself again:
First, I want to learn the correct way to design
Second, your drawing came ou after slicing with a face at one end of the sleepers where it hits the rail, see the read line at the lower bended rail. Sometimes a sleeper breakes off when having a face in between to parts of the print as the fill-in, when printing, does pass the red line. So I tried to correct that.

Meanwhile I reversed faces and it solved the problem, thank you.

SketchUp has to deduce which direction you want a new face to be.
A face has a front and a back and by default these have different colors set in the current style [Monochrome shows this even if you have painted them].
Drawing a flat face’s outer loop counter-clock-wise fixes its orientation ‘up’ - except if its z=0 when it always faces ‘down’ irrespective of the loop’s direction ! That’s because SketchUp assumes you’ll want to PushPull it upwards and that way the downward oriented face is retained when you pull its extrusion upwards.
Drawing multiple faces, overlaid etc can cause confusion as the complexity increases.
However, you can select any wrongly oriented face[s] and use the context-menu to reverse them so you see no backs - I recommend making your style’s back-face color very distinctive - say bright-blue, magenta or lime-green ?
It’s best to correct the orientation of faces as you model, using Monochrome mode etc, then applying materials later on…

Thank you for the answer, I was not aware of the face’s back and front side. Can you explain what is meant by the face’s outer loop? I tried to draw a square face using the line pencil tool, drawing the boundery lines clock-wise resp. counter-clock-wise but no difference in orientation.

The face’s outer-loop is indeed the edges that make up its outside perimeter.
A face can have an inner loop - e.g. if it has a hole in it.

The drawing direction of the outer-loop should affect the face’s orientation, BUT if the face is flat on the ground [z=0], then it is always made to face downwards - the loop direction is ignored - see the earlier explanation about subsequent PushPull etc…

BUT the important thing is that a face’s orientation affects how it extrudes with PushPull, and 3d-prints etc.
So just ensure that faces are suitably oriented and reverse any that look in the wrong direction…