How much can a hairline gap between surfaces affect sound control?-Acoustics Forum-Acoustic Fields Forum

I am working through the various pros & cons to reduce outside train sound getting through a window and could use some help.
My current preference is an exterior flush-mount 1/4″ laminated storm window. For water drainage and condensation control, the installation requires sitting the bottom of the window on the sill with no blind stop or caulking. In my case , the storm window would rest on a sloped brick sill.

If a properly installed laminated panel would get me to say an OITC=30, about how much of that value would be lost due to having that small gap without the benefit of a blind stop and caulking? If loss is significant, I would look into either a surface mount storm window that overlaps the rough opening in the brick wall or I would put together an interior window plug/panel. Thanks for any insights

PQ in east TN