Haverhill service ended completely back during the dark days of the commuter rail system. In a nutshell, once the MBTA began supporting commuter rail service, it was largely cut back to the MBTA service district (inside Route 128). The B&M continued one single commuter trip (via Woburn/Wilmington on the Lowell Line) until 1976, when all service beyond Reading ended. It wasn't until about 1979 that service was restored, this time operating via Reading.
For a long time, weekday service was really set up so that only every other train ran to Haverhill - alternate trains short turned at Reading. I don't recall if there was ever a similar set up for weekends, but keep in mind that all MBTA weekend commuter rail service is pretty sparse. Haverhill is certainly the worst of the north side lines in weekend frequencies, but it's not far behind the Fitchburg Line. Every 2-3 hours is about the norm - it's just not like NYC, NJ, Philadelphia in that regard.
Wondering if I'll see the Haverhill double-tracking finished before I retire...
Photo: Melbourne W7 No. 1019 on Route 78, Bridge & Church Streets, Richmond, Victoria. 10/21/2010