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Mr Downtown

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About Mr Downtown

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  1. cab signals

    Setting aside the snark and complete guesswork offered in this thread, I talked with some experts and I think I understand better now. I was having trouble understanding how the blocks could be isolated from each other without some kind of electrical insulation. While it's technically accurate to say there aren't insulated rail joints between blocks, there are instead "impedance bonds" that prevent the audio frequency signals from traveling between blocks but do pass the 600VDC traction power. Isolating high-voltage running rails requires big hefty nonconducting blocks while low-power AC signals—not unlike what a big stereo amplifier sends to the speakers—can be blocked more easily. So at each block limit, the logic board looks at whether a train is occupying the block(s) ahead and injects the proper signal: yellow-35 mph, yellow-25 mph, or yellow-15 mph. If a train enters an occupied block, where its leader is shunting (shorting out) the injected signal, no signal will be received and the cab signal will "fail" to red.
  2. cab signals

    Huh? The same frequencies are used systemwide. And if there are no insulated rail joints, how does the train keep from shorting out the very signal it is receiving?
  3. cab signals

    Well, that also puzzles me. I'm pretty sure CTA uses fixed blocks, not the moving blocks possible under more modern systems, so it's not based on "so many feet behind the other train." In addition, Bill writes "CTA uses a high frequency cab signal system which functions without insulated rail joints separating the blocks." How exactly would that work? How would a train only pick up the signal from the block it's about to enter, and not from the next one ahead?
  4. cab signals

    I'm trying to better understand CTA's cab signal system. My understanding from Bill Vandervoort's wonderful web page about it is that the cab display shows four aspects: green, yellow, red, and flashing red, indicating the condition of the block the train is just about to enter. But traditional wayside signals, as I understand it, showed not only the condition of the block about to be entered but also the block beyond that so that trains could slow before entering an occupied block. I guess my question is whether this is a significant difference. As I understand it, CTA trains get a yellow-35mph signal when the block beyond is occupied, and this is presumably done by the occupied block sending its condition back a block so the circuitry would show a yellow rather than a green. But that same logic could have been used for wayside signals, where instead it was thought necessary to show two blocks ahead. Was this simply a judgment call by CTA in the 1960s that their braking was sufficient to not need the same advance warning that steam roads traditionally used?
  5. Lasalle Street car

    The tunnels predate even cable cars by two decades. The Washington Street tunnel opened in 1869 and LaSalle Street in 1871. These were merely pedestrian and vehicular tunnels, occasioned because the bridges were open so frequently. They had fallen out of use when the cable cars were introduced, and proved the perfect solution to the bridge problem. In 1891-92 a third street railway tunnel was built just north of Van Buren Street. After the Chicago River was convincingly reversed in 1900, the water flow scoured the riverbed and exposed the tops of the LaSalle and Washington tunnels. In a remarkable bit of engineering legerdemain, these were rebuilt at a lower level by lowering the floors and then the roofs, keeping them watertight all the while. As for the precise locations of the streetcar tunnels, the Washington tunnel lay under Washington Street, with with center entrance portals east of Clinton and west of Franklin. The LaSalle tunnel lay directly under LaSalle (a little east of the modern centerline), with portals north of Randolph and north of Kinzie. The "Van Buren" tunnel was 120 feet north of Van Buren with portals east of Clinton (under the L platform) and west of Franklin. That portal in LaSalle south of Kinzie is just a connection down to Carroll Street. The streetcar tunnel portal was in the next block, north of Kinzie. The south end of the LaSalle tunnel was cut off when the subway was built under Lake Street in 1939. The Washington Street tunnel closed in 1954. Its west portal was visible underneath the North Western Station trainshed viaduct until it was decked over in the 1980s. The Van Buren tunnel was closed to regular traffic in 1924, but was used for training and emergencies until 1952. Both portals have been sealed, but there have been occasional discussions of using it as part of a pedway link to Union Station. A bit about the tunnels from the Encyclopedia of Chicago. An interesting article about the tunnels is Piehl, Frank J. "Our Forgotten Streetcar Tunnels," Chicago History, Fall 1975. More detail about the 1910s lowering can be found in Artingstall, William. "Chicago River Tunnels—Their History and Method of Reconstruction," Journal of the Western Society of Engineers, Nov. 1911.
  6. IRM CTA bus route maps

    Andre, do you (or someone else) have the Route Descriptions chronology in any computer format? That's what seems to have been lost. I have a 1997 printout, but it would all have to be retyped or OCR'd to get it on the Web.
  7. Lake Transfer station history

    Here's the 1916 Sanborn's fire insurance map:
  8. Clark/Division Red Line Rehab

    I'm not sure why Harrison got a unique design, as the current city standard is this black pseudoVictorian design, installed in recent years at Dearborn/Jackson, Congress/LaSalle, and even at the Polk end of Harrison. The new Harrison/State kiosk is too close to the curb, and the plate glass on the west sidewalk entrance is regularly scraped and shattered by truck or bus mirrors. About half the time it's a steel-and-plywood design.
  9. Pace Bus App

    The app known as Transit seems to work just fine with CTA and Pace, as well as systems in other cities you travel to.
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