The Boston & Maine had a "Holding Signal" in its rulebook. There was one at Parker Street, South Acton, MA on the Boston-South Acton-Ayer route. It was installed after the line was single tracked from South Acton to Willows about 1966. If the westbound interlocking signal at South Acton displayed a stop inidication, freight trains exceeding 60 cars had to hold back behind the Holding Signal so as not the block the Parker Street crossing. Shorter freights and passenger trains could pull up to the South Acton interlocking. The signal was automatic and conveyed the indication of the South Acton interlocking. Trains that did not have to Hold could proceed at a speed governed by whatever the automatic signals displayed, probably approach medium, approach and then stop. They were position light signals and in 1968 they were also located at Ballardvale, MA southbound, Newfields, NH northbound, Worcester, MA and southbound Saco,ME.