There was a 2-story black corrugated-metal-sheathed structure on the SW corner of the Grand Junction crossing of Mass. Ave. in Cambridge by MIT, and back in he late 60s on a Sunday, I did see someone exit and lower the gates for a movement.
Also, wasn't there a crossing tender's cabin that was part of the Erector-Set-like structure that long ago used to be over the tracks at Ayer?
On the subject of the Concord Street, Framingham, crossing, I can post with some knowledge, both of Arthur "Mac" McKenney and the shanty since I took the Middlesex News photo linked by stivigi (Thanks!). Arthur started off as a freight handler in the freight house which was where the present Track 2 T platform is. When that job was abolished, he bid on the crossing tender's job, and when that job was abolished, he was assigned to go on foot patrol and pick up scrap. Instead, he picked up a pen and put in his papers. He was one of those who took working for the Central, even the Penn Central, and Conrail as a solemn duty. The shanty was barely big enough for him, the phones, and the approachbell equipment, but it would also occasionally get turned into a block station with a resident operator. The last time I recall was when welded rail was being installed east of CP-22 and the dayside operator was an ex-LV trainmaster trying to hang on for his RRA. I wasn't there when the shanty was removed, but I don't think it would have survived being transplanted.
"A gray crossover is definitely not company transportation."