on the Philadelphia–Harrisburg section of the Keystone Corridor. Both converge at Zoo Junction near 30th Street Station in Philadelphia. All were built by the former Pennsylvania Railroad and are now maintained by Amtrak. Boston, Massachusetts area An abandoned underground junction on the Tremont Street subway approaching the Pleasant Street incline The B branch splits off from the C and D branches of the MBTA Green Line via an underground flying junction just west of Kenmore station. The Union Square spur splits off from the Green Line Extension of the MBTA Green Line via an aerial flying junction on the Red Bridge viaduct in the Inner Belt area of Somerville, Massachusetts just north of Lechmere station in Cambridge. Lead tracks to the GLX maintenance facility also split off from the junction slightly further west. The two southern branches of the MBTA Red Line in Boston split via a flying junction just north of JFK/UMass station. In addition, lead tracks to Cabot Yard maintenance facilities branch off from the junction. Chicago, Illinois On the Chicago "L", where Orange Line trains diverge from Green Line trains north of 18th Street, as well as underground where a non-revenue flying junction separates Red Line trains heading to 95th from those heading to the South Side main line, currently used to send some rush-period Red Line trains to Ashland/63rd. The Milwaukee–Dearborn subway (now part of the Blue Line) was constructed to have a flying junction where turning between Lake Street and Milwaukee Avenue at Canal Street. The outbound tunnel and its stub, designed to continue west under Lake Street, was bored at less depth than the inbound tunnel and its Lake Street stub, in order to allow future Lake Street trains (now part of the Green and (Pink Lines) to run under or over the opposing Milwaukee Avenue trains while entering or exiting the shared portion of the Lake Street tunnels. Plans in 1939 called for tunnels to replace the elevated Lake Street tracks east of approximately Racine Avenue. By 1962, the planned Lake Street tunnels to/from Racine Avenue would have curved south to Randolph Street and bypassed the Milwaukee-Lake-Dearborn tunnel entirely. Another flying junction is under construction immediately north of Belmont/Sheffield to increase capacity on the Red Line, Brown Line, and Purple Line Express. Denver, Colorado On the Regional Transportation District in Denver between the Southeast Corridor and the I-225 Corridor: the Southeast Corridor is