Due to obscure nature of MS technologies and protocols and unknowns regarding your system setup, rails version etc, I kinda have a feeling you might not get the answer you are looking for.
So, if this unwanted case happens, let me give a couple of alternatives...
Alternative 1
Set up a web server on the SMB host to serve the files, mount the folder in apache and stream them over HTTP directly to the client without the webapp as the middleman
Alternative argh :-(
If all else fails,
considering you have a relatively small number of users and high network latency it might prove a decent enough choice to simply stream file contents using regular File.open
(How do I read/write binary files?) and Rails' inbuilt streaming (this will depend on your rails version, Rails 4 should handle this well, Rails 3 is doable too)