Doh glad to hear you solved the problem. I was looking at the images and you were sure to mention that all the curves were connected.

Not having all the curves connected is one of the top reasons birail will fail. Also remember to rebuild your curves (or check the attributes and make sure they have uniform parameterization the start and end spans are whole integer values). As you cut and reconnect curves the parameterization can become non-uniform and this will create problems.

Also be sure the rails have equal spans and again that they have uniform parameterization.