Plans filed for all affordable, 12-story residential building in Chinatown

There are some deeply detailed discussions on that architecture/urbanism board regarding this very idea. Essentially, there would be a junction designed to allow Nubian cars to go either to the Seaport via South Station, or to Park St and beyond. Likewise, cars from the north would have the option of going toward Copley, Nubian, or the Seaport. It's all very doable, but as Ari notes, the key requirements of funding, political will, and neighborhood buy in are not currently there.

Should be light rail.

Heavy rail is Red-Orange-Blue. The cars are wider and can't take sharp curves, the trains are longer, and they operate off of third rail power, which would be next to impossible in a shared right-of-way (there are third rail power crossings in a few places, notably Long Island, Westchester and Chicago, but the T wouldn't want to do this and probably for very good reason). Plus, the tunnel which extends south of Boylston was built for streetcars, which would preclude Red Line-sized trains (Orange and Blue line trains are sized for streetcar tunnels since they originally were streetcars; in fact the Orange Line used this very tunnel from 1901 to 1909 when its own tunnel was complete). They also have higher floors, so level boarding from the street would be more difficult. Light rail is basically Beacon and Comm Ave, which would look quite nice along BHA (with, you know, some trees, although the same could be said for Comm Ave in Allston, too).

The tunnel from South Station to the Silver Line was a complete white elephant which was rightfully killed off. Buses in tunnels just don't work, they require much wider tolerances, operate at lower speeds, and given the amount of water present under Boston, the pavement doesn't last well, especially since buses have very heavy axle loads so they require a lot of concrete. But then at Boylston, because the existing Green Line tunnel is stacked over the old Green Line tunnel, the bus tunnel was going to have to go three levels down, and then make some kind of loop under the Common, with all sorts of environmental and historic issues there, to get to the second level. And no one could even explain why a slow bus ride underground to get from Nubian to the Airport was worth $1.5 billion. The bus lane on Essex Street and the transfer to the Silver Line there made just as much sense for less than a penny on the dollar. Even better would be to put the Silver Line on the surface in a bus lane on Summer which would be way faster than the underground loop-the-loop. The tunnel there was originally designed for light rail, which would be far better at handling the crowds on rainy days on the Silver Line (which is at capacity when it rains or is cold out because people will generally walk to South Station which is faster, but take the bus when the weather precludes this).

tl;dr light rail

BUT seriously... Is the round building at one corner of the park still a church? I went there once after St. Phillip's on Harrison Avenue burnt down on a field trip from the "temporary" sanctuary that replace it. I finally settled down at Immaculate Conception Church also on Harrison Avenue, As I recall the round church was truly multi-denominational.

BOT: That design looks ultra-cool. I wouldn't mind living there.