Did you know that you can earn a bike? Imagine, getting where you want, without the overcrowded bus. And lets face it, the MBTA is built for the Monday-Friday work community. A well maintained bike is great 7 days a week, any time you're ready to go. And why not use both? Except during the prohibited hours, you can take your bike on the train. There isn't a time restriction when using the bus,there might be a space restriction as buses only allow two bikes to travel on it's racks at a time. I found a very helpful Instructional VIDEO.
Also, your Charlie Card can be useful in that instance, if you've registered your card online to gain access to the MBTA Bike cages, then you can lock it safely and use the train.
They maintain the bikes. You ride.
If you don't want to commit to maintaining a bike, or you want another flexible option for instances when you don't have your bike, consider a bike share. Blue Bikes and Lime offers several plans, even a low-income plan. Contact them for more details.