I wish Boston would put one (or a 4mph one) underground walking path between Bowdoin station and MGH so that people could easily transfer without having to either clog the green line or cross traffic in the rain/snow.
The DC Metro talked about putting tunnels between some close downtown stations so people could walk rather than take trains for close transfers. For now, they have programmed the electronic fare system so that if you walk between the stations on the street you don't pay twice.