Once a rail is powered, if you try to extend the signal by adding a redstone signal in one of the already powered rails it won't work, you'd have to disconnect the one you are gonna power from the original source, power it and then connect it, but then if you remove the second power source it will remain powered.

I can assume it has something to do with the signal update on powered rails, since the bug apparently consists on powered "powered rails" not being able to change it's signal strength once powered until they are totally disconnected from a power source.

I hope it makes sense, I feel I blabbered quite a bit there...