Adding another power source to a line of powered rails does not update them.

What I expected to happen was for the extra power source to update the rails further down the track, making them powered.

What actually happened was that the powered rails further down the track did not update, and stayed unpowered despite the proximity of the power source

Steps to reproduce:

1. Make a long line of powered rails
2. Place a lever at one end and turn it on, ensuring that at least some of the other end of the track is unpowered
3. Place another lever halfway along and turn it on
4. The powered rails further along the track do not update