Redstone can change the direction of a rail junction even when it is separated by a block of air. I would expect redstone to only affect junctions directly adjacent or separated by a single solid block.
Furthermore, the redstone only changes the junction's direction once. Toggling the redstone power will not continue to affect the track after the direction has already been changed. (See attached image.)
It seems that this only happens with redstone wire. For example, if a standalone lever is two blocks away from the junction, toggling it has no effect whatsoever on the rail.