I've never had a card get underneath - not even a little. The table is padded all the way to the end, so the rail sits and ever so slightly presses into the foam, effectively creating a seal.
I need better pics, but this was a cellphone shot from 2012. Here you see the padded playing surface. On the right you can see the underside of the rail ready to go on.
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The rail consists of 2 bits of wood. One is from a sheet of plywood with an oval cut out of the center (creating the ring). The second is a 1.5" cut from the circumference of the playing surface, which is screwed to the ring, forming the rail. When the rail is set onto the playing surface, the outer circumference surrounds the play surface from whence it was cut. The only possibility for horizontal movement is the distance of the width of a jigsaw blade - and that is filled in by the fabric of the table, the table foam (it wraps under the play surface), and the fabric on the rail.
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