I will try to explain.
1st, You can create splines in any viewport You want. If anything comes out wrong- we can always fix it. "Reset Xform" helps most of the times.
To understand it simply and logically I can state like this- You draw X spline in top viewport along x axis. Draw Y spline in top viewport along y axis:
Now let's swap straight X spline with circle:
Weird stuff happens, but do not worry- seems like Y just was too long to understand what is going on:
As You can see, we can also reverse Y spline to change starting direction. All is simple for now, right?
Let's increase curve steps for smoother segmentation and now we can lift End vertex of the Y spline too see the magic happen:
Then all is left just easy final tweaks:
Now let's see why Your spline does not work, and why Tutorial spline does work.
Let's start with iToo vertical spline. To see what RailClone "sees", please Add reset Xform modifier and delete it
You can paste this line in MaxScript listener or do it manually as show in the top of the post and delete it:
resetxform $; deleteModifier $ 1
Now let's do the same on Your Y spline and reveal what RailClone "sees" as Y spline:
Good thing is- now we can understand how to fix it (in spline sub-object level):