I'm stunned. I had sent a quick letter to the Orange line chief - who probably doesn't really know the dilli-o - cc'd the GM, of course, and got a response from engineering.
From the T:
Thank you for your interest and for taking the time to write to us
regarding the new LED signs.
The new signs are part of an on-going project which is updating our PA
and LED announcement capabilities at key stations. The signs you
mention will provide service-related information, such as approaching
train information, next train designation and service announcements.
The information will begin transmitting in early April.
Thank you again for your inquiry and for your continued support of
Infrastructure & Engineering
500 Arborway, Second Floor
Jamaica Plain, MA. 02130
617 222 1820 - Tel
617 222 3918 - Fax