Thorndale Tower
Thorndale Avenue and Broadway, Edgewater

Service Notes:

Red Line: Howard

Quick Facts:

Established: 2021
Original Line: n/a
Rebuilt: n/a
Status: In Use



As part of the CTA's Red-Purple Modernization Program (RPM), a new interlocking was planned to be installed at Thorndale. RPM's Lawrence to Bryn Mawr Modernization (LBMM) project includes replacement of the four-track embankment between Leland Avenue (south of Lawrence station) and Ardmore Avenue (between Bryn Mawr and Thorndale stations) with a new elevated structure, which would be built in halves (referred to as Stage A and Stage B by the project) so that two tracks can remain in service through the construction zone at all times. To achieve this, new interlockings are needed at the north and south ends of this zone to merge and sort trains transitioning between the 4-track and 2-track zones.

The interlocking at the north end of the LBMM area will be at Thorndale Avenue, south of the station. The new interlocking includes three diamond crossovers -- a pair between tracks 1 and 2 and between tracks 3 and 4 at the north end of the interlocking plant, and one between tracks 2 and 3 at the south end -- and a new tower.

The plan also includes the decommissioning of the existing Granville Interlocking and removal of the interlocked switches as well as the existing hand-throw switches at that location.

Construction to install the new interlockings at Montrose and Thorndale each had its own set of challenges. At Thorndale, because the interlocking is on a ballasted roadbed on a solid-fill embankment, there was slightly more leeway in placing the track compared to the open-deck steel elevated structure at Montrose, with the ability to using re-grading and tamping to fix any slight variances encountered in vertical or horizontal alignments. However, running the traction power, signal and communications systems cabling under the tracks required excavating and installing underground ductbanks between and across all four tracks connecting the relay house and substation on the west side of the right-of-way to tracks, which was very involved work, especially in a 100-year old filled embankment.

During the period from mid-July to mid-August 2020, the ductbanks were excavated and installed, as were the three new diamond crossovers -- this work occurred during a four-week series of consecutive reroutes on the north main line. Each phase of these reroutes shifted service between tracks to provide the necessary access for the RPM contractors to install the new trackwork. The reroutes began at 10pm, Friday, July 17, and last reroute in the series ended at 4am, Monday, August 17.

During this period, the 4-track main line was reduced to two tracks, resulting in Purple Line Express trains running on the same tracks as the Red Line between Granville (or in some cases, Howard) and Belmont on weekdays; however, only the normal express stops were made. Running the two services together caused relatively few delays to service, as service levels on both routes had already been reduced and interval between trains widened due to depressed ridership levels caused by the COVID-19 pandemic and resulting increase in people working from home or otherwise altering their travel patterns. For two-week periods, Red Line trains bypassed Granville, Thorndale and Bryn Mawr stations in one direction or the other (Howard-bound service bypassed those stations July 19-31; 95th-bound service bypassed August 2-14). On the first, middle and last weekends, trains bypassed all three stations in both directions, temporarily closing them for the weekend, with a bus shuttle between Loyola and Berwyn providing replacement service. Temporary track connections between tracks 2 and 3 north of Thorndale and between Thorndale and Bryn Mawr -- installed at the start of each phase and removed at the end of each phase, during reroutes causing the aforementioned weekend station closures -- allowed the number of affected stations to be minimized.

The "extended reroute" approach to the interlocking work reduced the number of days in which both passengers and the local community were impacted by construction by exchanging shorter-duration weekend reroutes for a longer reroutes that included weekdays. Similar to the CTA’s approach for the Red Line South Reconstruction Project in 2013 that rebuilt the Dan Ryan branch’s track system from the sub-grade up, it “ripped off the band-aid” and reduced overall impacts while improving contractor efficiency by reducing time spent mobilizing and remobilizing equipment and staffing over successive weekends.

The extended reroutes installed the new trackwork and some other associated infrastructure for the two new interlockings, but they were not functioning, tower-controlled interlockings at the end of each extended reroute. There still remained much work on signal, communications and power systems at each location, and commissioning activities to complete. This was true not only at the new interlockings, but along the whole route from Montrose to around Loyola, all of which received new bi-directional cab signaling to support the 2-track operation and provide new, more stable signal infrastructure for during construction. The new Montrose relay house was lifted and set on its platform at track level on Thursday, October 22.

Equipment installation, testing and commissioning work on that new signal system continued through fall, winter and spring into May 2021, with parts of the interlocking coming online as certain activities were completed. On Saturday, March 20 at 8pm, all of the wayside interlocking home signals (both normal and reverse movement signals) on tracks 1 and 2 were placed in service at Thorndale Interlocking. However, the related switches remained clamped in the normal position until the interlocking was fully tested and placed in service; the turnouts functioned as hand-throw switches, and could be operated by Power and Way Maintenance Signal Maintenance personnel as needed. A week later, at 8pm, Sunday, March 28, the remaining home signals were placed in service, with the same limitations on the use of the associated switches.

On Friday, May 14, 2021 at 10am, Thorndale Interlocking was placed in service, with all switches and signals available for use from local tower panel.

RPM LBMM Stage A construction, when 2-track operation began between Montrose and Thorndale interlockings, began at 12:01am, Sunday, May 16, 2021. Tracks 3 and 4 closed, and all trains used track 1 (southbound) and track 2 (northbound) between Montrose and Thorndale.

Installing, testing and commissioning work for additional functionality at Thorndale Interlocking continued through May and into June, including the ability to put the interlocking on automatic during off-peak periods and have routes automatically established for Red Line trains, and automatic vehicle identification (AVI) to detect and differentiate approaching northbound Red Line trains from Purple Line trains and automatically establish northbound routes from Track 2 onto Track 3 for Red Line trains and onto Track 4 for Purple Line trains.


This article is a stub. It will be expanded in the future as resources allow.