The single track section through Waltham dates back to the late 1950s and was necessitated by close clearances in the cut east of the station. When the Boston & Maine started handling piggyback and other large dimension freight cars, the line was single tracked to make more room. After all these years it surely would have been cheaper to enlarge the cut and maintain double track, given the cost of the switches and singnals and delays to trains over the decades, but that's not the decision that was made back then, and these days the cost would probably be prohibitive.