I am not using mods; this is a vanilla version of Minecraft. It seems my client is the only affected one, and starting a new world actually doesn't fix it. I have no idea what to do either.
Just as I post this, I fiddled around a bit more with the light glitch and it suddenly resolved itself! No further help is needed.
I tried what you mentioned as far as the incline, but I see no difference between the incline start or the flat rail start; the glitch still remains to both.