Signal Power Save Mode

I am sure that many of you are familiar with "Approach Lit" signals, where the signal lamps only light up when a train is in the block approaching it (or sometimes, in CTC territory, leaving it).  This feature, still employed today, was originally used when block signals were lit by consumable battery power and hung around to conserve bulb life as well as battery power in times that pole line power failed (which was often).  Well it turns out that there is in fact a third option between leaving signals on at full brightness all the time or only lighting them up when a train is approaching and, at least as far as I can tell, it was a Conrail thing.

I first noticed this at CP-103 on the now-Amtrak Hudson Line where one of the 4 approach lit signal heads maintained a low level glow, in this case of the red variety, with the rest remaining completely dark.  

Of course when a train approached they lit up with their full intensity.  Strangely, the southbound masts did not exhibit the same type of power save feature, with all lamps remaining dark.

I thought this was a one off until a few weeks later when I noticed the same effect on the eastbound signal at CP-BLANDON on the former Conrail Reading Line. 

While the Hudson Line was re-signaled in the 1970's or early 1980's, CP-BLANDON was re-signaled in the late 1990's, so this is clearly a feature that was being employed for a number of years.  However it is not all together clear why it is used to selectively.  If I had to guess, the one similarity I noticed between CP-103 and CP-BLANDON was that a power save mode signal was located just past a grade crossing.  The ghostly red light may serve as a visual reminder of the interlocking limits to hi-rail vehicles entering or exiting at said grade crossings.  High rail vehicles and other track cars might not shunt the track circuit and therefore would not light up regular approach lit signals.  A burning red marker light would go a long way to preventing careless Stop signal violations.

Anyway, if anyone knows the true reason for this feature please let me know in the comments.