Bottom track. Need top n bottom

A quick learner from California says:
Is the idea that the wheel will glide upon the vertical portion of this lower track? I was thinking (as I saw a neighbor's track replacement) that the lower track would be U-shaped to keep the screen from coming off an open lower track like yours. Am I looking at it incorrectly? Is the vertical portion of your lower track the outside edge to prevent described above?

My complex has the old tracks wherein the screen wheels hang off the upper track. These are not the everyday screens from the big box stores. Replacement screens of this kind are coming in at $375-420 in bids. OUCH!

So I am hoping I can simply put on new tracks and get me a screen door at last,
I look forward to your response.
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Tom from SWISCO responded:
Thanks for posting. With our 80-111 Bottom Sliding Door Track, the concave wheels would ride on the knob at the top of the vertical "wall," not along the flat "floor."
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