While following the mine cart tutorial I was unable to create the rails using the curve tool as instructed. I applied a shape made out of one curve to another curve as a BevOB as shown in the tutorial. However this did not produce the desired result and instead gave me a mess of polygons 
.I have attached 2 pictures showing before and after I use the BevOB.
I am new to blender and have only been using it for a week so I am probably just making a noob mistake. Also Here is the link to the mine cart tutorial I was following.
Two things it might be:
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Which curve did you assign the bevob to? try switching them.
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Try scaling the bevob down.
Quick tut on bevob.
- Press 1 for front view.
- Add Curve Space>Add>Curve>Bezier.
- Press tab to go inside, then v for vector.
- Select the middle of the left curve point and E to extrude then z to constrain it and move it up a bit.
- Now press E then x and move it over so it is on top of the first curve point on the right.
- C to make it cyclic, and you should have a little box. Move it over to the left.
- Space>Add>Curve>Bezier and just scale it up a bit.
- Select the box curve we made, Hover over its name in the menu CTRL C to copy
- Select the new curve we made, go to edit menu and CTRL V to paste into bevob field.
- Now scale the box curve so it looks right. You may need to select it and in edit mode use the menu at the bottom left of the window Curve>Segments>switch direction.
Whatever you do, keep playing and Keep Blending! 
Attachments
curve.blend (122 KB)
I was able to duplicate your curve tutorial no problem. However i still could not get the mine cart one to work so I recreated my curves following your advise and now it works. I must of done something wrong the first time I created the curves.
Thanks for the help 
.
Glad I was able to! See you around.