Two of the most thought-provoking books I've read are Freakonomics and its sequel. Although they focus mostly on economic ideas, they also explore some unintended effects of public policy. Besides my human appreciation of irony, I find it fascinating to see how one thing can cause a chain of events. As a future engineer, it's important for me to see how one decision I make can have effects down the line.
Over the last century, engineering standards for trains have increased in a similar manner to how those for cars have increased. Just as manufacturers can no longer make Pintos that blow up when they get hit from behind, they can no longer make wooden trams or railroad cars that cannot survive a collision with an automobile. Today's modern light rail vehicles (colloquially, trams and trolleys), for example, are significantly heavier than the PCC streetcars that dominated during the 30s to the 70s. (The PCCs weighed just 35,000 to 42,000 pounds; 70s-era Boeing cars weight 67,000, and modern Type 8 trams weigh 85,000 pounds empty - and 130 passengers can add 20,000 more on top of that.)
Although fuel usage is not an issue for electric trams, weight can still be an issue. Should Green Line cars ever use the Pleasant Street Incline in South Boston again, the flyover ramps would have to be rebuild for the heavier modern cars. It is for this reason that the PCCs are still used on the Ashmont-Mattapan Line: the three bridges on the route would have to be rebuilt from scratch to accommodate the newer trams, and they would not be able to get an exemption to use lighter European trams because the line has grade crossings where a tram could conceivably hit a car. There's also no sense in abandoning reliable old cars when there's still a shortage of new cars.
Engineering standards, particularly crashworthiness standards, also affect mainline rail operations. Speeds are limited on many lines because in order to run above 70 miles an hour though grade crossings, the first car must be a locomotive or unoccupied car. FRA (Federal Railroad Administration) rules require certain steel side beams for operation above 125 mph. These side beams are located right where fold-down stairs go, so the high-speed Acela has no stairs - and can only stop at high-level (4 feet high) platforms.
Heavy modern steel railroad cars also require more energy to slow down, and to speed up, than the light wooden cars of years past. Again, this poses little problem with electric locomotives (or self-propelled electric railcars), but only the Northeast Corridor plus (most of) the New York and all of the Philadelphia commuter lines are electrified. When, like Boston, all of your commuter lines are diesel locomotives hauling 4 to 8 cars, it is impractical to have stops less than about 2 miles apart. Although their are some exceptions (Melrose, Needham, and Dedham have 3 stops within 1.5 linear miles, and West Roxbury has a 4-stop cluster), the trend is for fewer, wider-spaced stops, particularly on the newer lines.
During the nadir of rail travel - from about 1950 to 1980 - many lines were abandoned, or service was reduced to starvation levels. To reduce maintenance costs and to save on fuel, many stations were closed even on active lines. The Worcester-Framingham Line, like others, saw a number of its stations closed, particularly in the inner belt. Stations at University (BU), Allston, Brighton, Faneuil, and Newton were closed, leaving the Allston-Brighton area devoid of good transit options and heavily car-dependent.
The Allston depot was built in 1887 by the Boston & Albany Railroad, replacing the 1868 "Cambridge Crossing" depot. Although it closed at an unknown date, it still stands where Cambridge Street crosses the Mass Pike. It was once Sports Depot restaurant; now it's Regina Pizzeria at the Depot. (It housed a steakhouse even when it was still operational.) I took a walk two weeks back, to explore Allston and to photograph the building. With permission from the manager, I took some shots. It's a fairly large building, and the first two are panoramas I stitched together.
The saga of Allston Depot is not yet over, though. CSX is leaving Beacon Park Yard (the large rail yard between BU and Harvard) and moving those operations to Worcester. This will permit the MBTA to run more trains on the Boston-Framingham segment with less freight interference. Harvard is calling for the establishment of a new stop in Allston, and money talks. If the MBTA can purchase the tracks in that area, then they may well remove or relocate one to make room for a platform. The favored site is under the Cambridge Street bridge - exactly where the stop was first located 143 years ago.