Northbound/Southbound Sign at Park

Theres a red sign hanging from the ceiling at Park Street's Green line outbound (Westbouond) platform. It lights up inside. Sometimes it glows, usually it doens't. What is it?

My guess is it has (had) something to do with announcing trains.

Here's an idea. The order of trains coming down the central subway is known for example at Copley long before trains arrive there. The order is known as soon as trains file out of Park because it is single track. Why not have electronic signboards in stations lisitings the line designations of the next say, 5-7 trains.

I.E. at copley;

D, C, B, E, C, D, B

or, more likely how it really is.... D, D, D, D, B, B, B heheheheh
That way at least you know you have a while to wait for your C train, as opposed to getting hopeful and excited everytime you see headlights.

Theres a red sign hanging from the ceiling at Park Street's Green line outbound (Westbouond) platform. It lights up inside. Sometimes it glows, usually it doens't. What is it?

My guess is it has (had) something to do with announcing trains.

The signs are indicators showing the arrival and direction of Red Line trains on the lower level at Park Street. The signs are intended to signal to the operators of the last Green Line trains of the day, which must hold at Park Street for their connection to the last Red Line trains of the day, that the Red Line connections in both directions have arrived, and they may depart after all passengers have made their way upstairs and transferred to the Green Line.

Of course, the signs work continuously, not just at the end of the day. So if you see the Northbound sign lit on the Green Line level, you know that there is a Red Line train to Alewife in the station below.

Regarding the other idea, it is technologically quite easy to have a system like this. San Francisco does this exact thing - in each MUNI Metro station, there is an electronic display and audible announciator that indicates the destination of the next train, but also how long before it will arrive! I won't comment or pass judgement on why the MBTA does not have such a system...