Holding signals

The former B&M used to have "holding signals" at some locations. One was on the Andover side of Ballardvale station, and it consisted of two lunar lights vertically placed, and two red lights horizontally placed; a little like the Pennsy position-light signals. When it was red, it required a train of 50 car lengths or more to stop. I guess so as not to block the crossing at Ballardvale, I don't know. The crew was then to call the dispatcher on the phone. [Obviously before radio days!] In any case, there were a few others of these around the system. Are they still there, and functioning?

They still have them, but regular interlocking or block signals are use

Also CPF 244 in Dover and CPF 114 in Waterville. I guess all the interlockings on the Northern could be considered holding signals as well since all the power switches are out of service.

I have been told of the reason for Slab City. If a switcher is working in Ayer, and one in Fitchburg, or Loeminster, the moves can be lined toward each other. The interlocking sepatates them.
In N. Adams, a local can work on the main. A road job can start out of Hoosick Falls. If not for Williamstown. The local or the road job would have to hold until the other made it from Hoosick Jct. to N. Adams.