In general, the current set up MBCR has with the MBTA does not result in seasonal layoffs. MBCR manages and operates the MBTA's commuter rail system and sometimes is awarded some capital improvement work. However, the bulk to the capital jobs are farmed out to private contractors, currently those in favor with the Republican administration in Massachusetts. Most of the capital projects are financed primarily with Federal funds. The strings attached to the Federal funds are that if there are laid off railroad people, contractors can't work. This has come to be interpreted that if there are laid off commuter operator workers, contractors working on the commuter property have to leave the property even though they are dealing with the MBTA directly, not MBCR.
On the Boston and Maine and Guilford, when they had MBTA contracts, they only laid people off the freight part of the railroad which was not a recipient of Federal funding. High seniority freight people bumped into the passenger service when work wound down usually late November or early December. However, Mass. Bay Commuter Railroad's domain doesn't have allow such a scenario (and Amtrak's didn't either). So you should be safe.
Give it a try. You'll get a good starting pay, you'll get training. If a winter is severe, you'll get snow duty and as a trackman, no hours of service rules apply, you can be on continuous time. If a winter is mild, you'll distribute materials, clean up the shops and make sure the trucks are all shined up. Within a year or two, try out for foreman. You'll be rules qualified and when classes open up, you'll be in a good spot to enter a conductors' class. Or move through the Engineering Department (M of W). You can learn to become a truck driver, welder or a variety of other occupations.
I started with the Boston and Maine on 8/25/1968, worked the B & M into the Guilford years, went over to Amtrak in 1987 and Mass. Bay Commuter Railroad in 2003 and retired from there on 2/28/2005. I had an NHL job (No Heavy Lifting) in the office. But it was very rewarding. You'll probably like it too.