allow R5 Bryn Mawr locals to turn without occupying an express track. Paoli interlocking may be removed if the four-track configuration were to continue west of the station. Paoli station may be reconfigured with high-level island platforms serving all four tracks, as part of a new Paoli transportation center. Frazer interlocking may be reconfigured for turning SEPTA trains and as the point where four tracks become two. Electrification may be converted to use commercial 60 Hz AC power instead of the special 25 Hz single-phase AC currently in use, although this is doubtful due to the costs involved, lack of real benefits and dedicated 25 Hz hydro-electric capacity at the Safe Harbor Dam. The Safe Harbor Dam also generates electricity for the Northeast Corridor itself, the power going from the dam to the NEC (at Perryville, Maryland), via a pylon network. Between Paoli and 30th Street Station, most of the overhead electric wire and other electrification system components date back to the original 1915 electrification, although the 1915 substations have been retired. West of Paoli the electrification dates from the late 1930s, and west of Downingtown the system is still controlled by the original 1939 power dispatching office in Harrisburg, utilizing electromechanical systems. Construction progress The four-track section between Overbrook and Paoli is numbered sequentially from the southernmost track (number 1 track) to the northernmost track (number 4 track). Between March 7 and June 27, 2005, Amtrak worked on the number 4 track between Lancaster and Parkesburg, and from June 27 to September 2, 2005, they worked on number 1 track. A work gang with a track laying system (TLS) installed concrete crossties, new continuous welded rail, and new ballast, allowing for . The track layout at Lancaster station was simplified so that trains no longer have to take a diverging route to access the station platforms. Because tracks 2 and 3 have been removed, tracks 1 and 4 are the only tracks in this section. Between October 3, 2005 and mid-December, Amtrak worked on the number 2 track from Paoli to a point between Narberth and Merion stations. On March 20, 2006, Amtrak started working on the number 3 track, starting within Paoli interlocking and working east towards Overbrook. As of April 6, 2006, a track laying system (TLS) has completed work to approximately milepost 16.7. 110-mph service started on October 30, 2006 following completion of a $145 million