Hi Marc, I don't think there is a way to get across via bike bath only, but one option is to take Nashua Street, behind TD Garden and cross O-Brien hwy to the Boston side of the path, Nashua St is pretty low key from what I've seen and there's a park there before you cross over to the museum of science. You could then use the longfellow bridge ( I think it's still open to bikes/peds) over to the Cambridge side if you want to avoid the rather busy o'brien hwy. There is a bridge on the cambridge side going over to Charlestown, over the commuter rail line and under the Zakim bridge, but I don't think there is really direct access from north station with dedicated paths