I just dug out my December 1953 edition of The Official Guide, and here's what I found. Two daily round trips Boston to Rockland. No. 55 left Boston at 2:25 a.m, Portland at 7:05 a.m., Brunswick at 8:00 a.m., arrived Rockland 10:25 a.m. Train 57 left Boston at 4:40 p.m., Portland at 7:20 p.m., Brunswick at 7:58 p.m. and arrived Rockland at 9:30 p.m.
From Rockland, No. 52 left Rockland at 7:50 a.m., Brunswick at 9:48 a.m., Portland at 10:35 a.m., arriving Boston 1:15 p.m. No. 56 Left Rockland at 4:20 p.m, Brunswick 6:33 p.m., Portland 7:10 p.m. and arrived Boston 10:10 p.m.
In addition to these two trains, there was also a Portland-Rockland round trip leaving Portland at 8:48 a.m., Brunswick at 9:31 a.m. arriving Rockland 11:25 a.m. The return to Portland left Rockland at 5:20 p.m., Brunswick 7:13 p.m. and arrived Portland 7:55 p.m.
In addition, the Guide shows three daily round trip buses between Rockland and Portland. There were also connecting trains to New York from Portland, but no direct through trains. At that time, there were still daily trains from Portland to Bangor, Farmington, Calais, Vanceboro, and even Montreal via St. Johnsbury.