A Logan Square train stops at the LaSalle station just two months after it opened, looking west from the LaSalle interlocking just east of the station in April, 1951. LaSalle was the end of the line until 1958 when the connecting Congress Line was completed, so trains turned through the diamond crossover to make the trip back to Logan Square. For a larger view, click here. (Photo from the Krambles-Peterson Archive)

LaSalle Interlocking
LaSalle Street and Congress Parkway, Loop

Service Notes:


Blue Line: Milwaukee-Dearborn Subway

Quick Facts:

Established: February 25, 1951
Original Line: Milwaukee-Dearborn Subway
Rebuilt: n/a
Status: In Use



LaSalle Interlocking was installed as part of the construction of the Milwaukee-Dearborn Subway, which initially ended at LaSalle/Congress station when it opened in 1951. Although the subway was intended to be continued west and LaSalle station was meant to eventually serve as a through-station, for the time being it had to serve as a single island platform, two track stub terminal. As such, a crossover had to be installed to control such a terminal, even if it was only meant to be actively used until such time as the subway was extended west.

The towerman at LaSalle Tower is working the panel, routing trains in and out of the terminal, in this view circa 1951. While more common at the time, shirt-sleeves, a tie, and vest gave way long ago as the typical attire for a towerman! For a larger view, click here. (Photo by J.R. Williams, courtesy of the Krambles-Peterson Archive)

LaSalle Interlocking is located immediately east of LaSalle/Congress station and controlled the movements in and out of the Congress Street Terminal station, as it was sometimes called in the mid-1950s. The plant consisted of a tower on the west side of the junction and a diamond crossover in approach to the two-track stub terminal. The interlocking -- a group of switches, track trips, and signals arranged to prevent conflicting train moves -- at LaSalle crossover was initially controlled manually from the control panel located in LaSalle Tower.

One of the CTA's first automated route-selection plants at a junction was installed at LaSalle Interlocking in February 1953. The selection programming worked like so: Inbound routing was based on which track was unoccupied, with preference given to the south pocket if both tracks were empty. Because the south pocket was, in fact, the northbound track (outside of the terminal, and would be once LaSalle became a through-station), trains in this pocket could depart the station and begin the trip to Logan Square on the northbound track without crossing over in the interlocking and thus did not risk fouling the line. Outbound routing was simply on a first-come, first-served basis triggered by an automatic train dispatching program in the tower about a half-minute before scheduled departure time. This departure program actually overrode the plant so that departures would not be delayed if a train arrived at the same time. The tower panel and necessary equipment was still left in place for manual operation of the interlocking when desired, such as in rush hour, emergencies, or other unusual circumstances.

On July 13, 1957, a severe storm struck Chicago, but the drainage system of neither the new Congress Expressway or the rapid transit line in its median, both under construction at the time, were not yet operational. With the Congress Line connecting to the subway just east of Halsted, a torrent of water rushed into the tunnels of the Dearborn Subway. Although the subway was equipped with automatic pumping equipment, it couldn't handle the flow. LaSalle is near the low-point of the subway (though the lowest is actually between LaSalle and Clinton stations, west of the interlockings) and so with the water level settling at Jackson station to the northeast, LaSalle Interlocking found itself underwater!!! The water was so high that its high-point filled half the interior height of LaSalle Tower and up to the archway soffit of the columns along the platform edges of LaSalle station. Service was truncated back to Jackson and LaSalle Interlocking was water-logged until days later when cleanup was completed.

On June 22, 1958, the new Congress Line opened for operation. Douglas trains were withdrawn from the Loop and rerouted to downtown via the Congress Line, which were both through-routed with the Milwaukee Elevated via the Milwaukee-Dearborn Subway, creating the new West-Northwest Route. At this point, LaSalle became a through-station and LaSalle Interlocking was no longer needed in daily scheduled service, since the station no longer acted as a two-track stub terminal. All trains now operated straight through LaSalle in both directions on their respective tracks. At this time, staffing was withdrawn from LaSalle Tower, although it still exists today.

The 1950s interlocking control panel remained in place until the signals and associated equipment were replaced circa 2007-08 as part of the Blue Line Traction Power and Signal System Upgrade Project. LaSalle Interlocking is still extant and is occasionally used for single-tracks and other special or emergency operations.


lasalle02.jpg (106k)
This detailed view of the original LaSalle Tower panel, seen on March 24, 1951, shows the machine when it was brand new and the latest technology in electrically-controlled interlocking tower panels. Note the clock and other details included in the panel face. Also note that the panel graphic includes the line continuing west of LaSalle and even includes Clinton station -- neither would exist for several more years after the panel installed and this photo was taken. The pack of cigarettes and books of matches sitting on the "next train" indictor box for the terminal are a reminder of how commonplace it once was to smoke even in workplaces. (Photo by B.L. Stone, courtesy of the Krambles-Peterson Archive)

lasalle-congress02.jpg (114k)
On July 13, 1957, a severe storm struck and Chicago, but the drainage system of the new Congress rapid transit line was not yet operational. A torrent of water rushed into the tunnels of the Dearborn Subway, settling at Jackson station. LaSalle was water-logged until days later when cleanup was completed. (Photo from the Krambles-Peterson Archive)