MBTA's Everything's Alright Signal

Those who find themselves looking down the MBTA Red Line's distinctive open span subway tunnels might be forgiven if they sometimes think their eyes are playing tricks on them. One moment a lunar white wayside marker signal is visible, the next moment it has seemingly vanished. Those with above average powers of observation might also notice that these signals are facing against the standard current of traffic and also vanish in the minutes before a train's arrival. These are worker protection or "TAK" lights for "Train Approach IndiKator" (also called "snitch lights") and their job is to illuminate when everything is alright, and extinguish when a train is approaching.  

Light on, all clear.

Light out, train approaching.

 

As one might have guessed these use track circuit state to indicate when a train is approaching in a failsafe manner. Placement of the TAK lights appears to have been around curves or other situations that limit the sight lines of track workers. They seem to have been designed for the older single direction ABS system and may only still be in service in the tunnels as the few I have seen on the above ground Braintree Branch appear to be bagged. 

TAK lights are one of those things that you never notice until someone points them out, then you can't help but notice. Back before the real time departure information they were a life hack to know if a Red Line train was approaching, but today, unless you are a track worker, they are mostly a neat bit of trivia.