Dont think the bridges will be an issue from Midway to Castlegar. You are probably thinking of a bit farther north in Myra Canyon. Major fires burned a bunch of bridges there in 2003. Repairs are in progress but probably have a ways to go and anyway they just wont be the same. I have ridden the line from Castlegar to Christina Lake. Really nice with some cool bridges and tunnels. As the other reply said, Google the KVR and you will find a link to a site that gives detailed but (I think,) confusing info.
I don't know if you will have enough time to find/order it but there is a book called Cycling the Kettle Valley Railway by Dan & Sandra Langford that covers your whole route. The ISBN is 0-921102-54-2 One thing to note make sure that you have a good light because there is a tunnel to go through and you can't see the light at the end of the tunnel(pun intended), I haven't done the trip but have talked with people that have and they say it a good ride.