Please take a look at the attached file.
In the first example, the rail is built off of a very small curve at the bottom of the model, and the large resulting surface flares out at the bottom, even though the rail curve does not.
In the second example, the rail is built off the very same resulting curve at the top of the model from the example above, yet the resulting surface does not match the previous small curve, rather it behave very erratically.
Why is this and what can I do about it?