New PTC signal heads, what will they mean?

[Michael Moroney]

At an entrance to an interlocking on a Massachusetts MBTA commuter rail
line also used for freight (Fitchburg branch) they've added a signal
head to each of two 3 high signals, at a lower level than the lowest
existing head, but not in line with them. They also appear to have a
white "S" painted on the back shade. So far I haven't seen this new
head lit, the PTC system may not be active yet. Since noticing these
heads are now facing trains (they were facing off to the side and
covered in black plastic when first installed, so as not to confuse the
engineer) I have only seen the other signals both displaying stop and
hold (all red) or dark. Can anyone either explain its meaning, possible
colors it may display or point me to a web page which does? Sorry my
Google-fu isn't working for this one and I can't find anything.

[Michael Moroney]

They also appear to have a white "S" painted on the back shade.

Correction: "C", not "S".

[Fishrrman]

Post by Michael Moroney
Correction: "C", not "S".

I'm not familiar with the territory, but my guess is that
the signal department is in the process of converting from
fixed automatic block signals (between interlockings) to cab
signals WITHOUT block signals.

The "C" probably means "clear to next interlocking". It's
used in other areas already.

This would be displayed for a train that had suffered a cab
signal failure, which would otherwise have to run at
restricted speed (since the engineer has no way to ascertain
the condition of the block ahead).

In order for the "C" to be displayed, there has to be an
"absolute block" (no other trains) from the affected train
to the next interlocking ahead.

The C means (the wording may vary from one railroad to another):
"Trains and engines without operative cab signals may
proceed on fixed signal indication, not exceeding 79mph, and
approach next interlocking signal prepared to stop"

At the next interlocking, the train must receive another "C"
(along with fixed signal indication) in order to proceed
further.

It's really only for when the cab signals on a train have
failed.

[Michael Moroney]

Post by Michael Moroney
Correction: "C", not "S".

I'm not familiar with the territory, but my guess is that the signal
department is in the process of converting from fixed automatic block
signals (between interlockings) to cab signals WITHOUT block signals.
The "C" probably means "clear to next interlocking". It's used in other
areas already.
This would be displayed for a train that had suffered a cab signal
failure, which would otherwise have to run at restricted speed (since
the engineer has no way to ascertain the condition of the block ahead).
In order for the "C" to be displayed, there has to be an "absolute
block" (no other trains) from the affected train to the next
interlocking ahead.
"Trains and engines without operative cab signals may proceed on fixed
signal indication, not exceeding 79mph, and approach next interlocking
signal prepared to stop"
At the next interlocking, the train must receive another "C" (along with
fixed signal indication) in order to proceed further.
It's really only for when the cab signals on a train have failed.

Thanks. Now that I had something better to search for, I found the same
text, as well as a couple of videos (on other systems) which show the
signal head active. It is a flashing lunar white.