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Saturday, June 25, 2011

METRA: Gresham Junction Tower

This is going to be the first in a series of inside looks at METRA interlocking towers in the Chicago area.  All of the behind the scenes photos that will be shown here were sent to me from an employee who had access to the towers in the time period 2006-2008 so don't go asking me how I was able to get into all these places.  However some of the sets will be interspersed with exterior shots that I have taken of the tower and/or interlocking.

Gresham Junction Tower, on Metra's Rock Island District commuter lines operating south of Chicago is a surprising survivor where conventional wisdom would have dictated its closure well before its actual demise on Jan 30th, 2010. The tower is also of special interest to the British signaling community because of the interlocking equipment installed within. For those of you who don't know, Gresham Junction housed unique Sequence Switch interlocking machine made by Standard Telephones and Cables Ltd of London. It was an NX design developed in the 1950's from Rotary system telephone exchange equipment. Not many were sold and Gresham Junction was the only unit installed in North America. The only other STC Sequence Switch interlockings anywhere in the world were two 1949 models installed in the old LNER boxes at Doncaster North and South and two improved models installed at Tollerton and Pilmoorin 1960 and 1961.

Gresham Junction is located at the northern split of the two Rock Island District sub-lines, which for lack of a proper name I will refer to as the Slow Line and the Fast Line. The Slow Line is used by most trains and has 11 stops between Gresham and Blue Island. The Fast Line is only used be a few rush hour express trains and has only two stops. Where the two lines re-join in the south there is the also still active Metra Blue Island tower, which has a more conventional NX board and is not to be confused with the more famous Blue Island tower nearby. As indicated by this line guide The Slow Line between Gresham Junction and Metra Blue Island is unsignaled, with trains operating under a Controlled Block system between the two interlockings. The Fast Line is signaled for bi-directional operation. In addition to the split Gresham Junction also handles a wye connection to an infrequently used connecting track that eventually winds up at the former US Steel plant in South Chicago near the famous Quad Draws at CP-509.

The tower is also unusual in that it is located such that a city street is between it and the Fast Line track it looks out on. In this photo we see the rear of the tower. The front can be covered by Google Street View. The tower has a typical slab side 50's design common to both towers in the Blue Island area.

Moving inside the tower we find a typical British green angled panel with some rather outsized NX knobs.Each know had a number of possible route selections.  All the operator had to do was turn the knob to the desired route and press in.  To cancel a route the knob would be popped back out. While the interlocking is clearly oriented North/South, the tracks of the old Rock Island carry an East/West designation in the timetable, so operators have drawn a handy compass rose on the panel. The line heading east used to sport 3 tracks, but the topmost one is now just a stub. Yes, there is a bit of an odd bottleneck on both lines where trains in both directions must squeeze through a single path. You can see where another turnout used to be located between the eastward slow line track and main track 2 and also that there were 3 tracks in the west direction as well. The tower still supports a double slip switch that all slow line traffic must negotiate. Finally note the blocking tag on the entrance button to the Westward slow line, which is probably in place for the manual block protection.

On the right side of the panel we see more of the same including a second compass rose. The wye track used to have a controlled switch at its east point, but that is now hand operated.

Pulling back to a wider angle we see the photo probably dates sometime after 2006 given the date on the retirement plan announcement taped to the panel. The company issued 2003 calender is quite a bit out of date. In case of Nuclear War we can see that this tower is equipped with ample supplies of water and paper towels. While the panel has seen a typical number of "deletions" over the yeard, its interesting how much room there was on the "west" side of the board. I am unsure if there were plans to add more interlockings or signal the slow route, but I do not believe anything was ever removed from that section of the panel.

Moving into the locking room we find the guts of the interlocking in the form of rotary circuit switches derived from telephone exchange technology. The Rotary System was developed by Bell Labs at about the same time that they were developing panel switches, but ultimately panel was adopted for the North American market, while rotary was sold in Europe.  I guess one can call it ironic that such switchgear would be imported back into the United States in the form of a railroad interlocking machine.

Here are selectors 1 through 5 and its astonishing how well maintained the equipment is. It is a testament to the original manufacturer and its subsequent maintainers that the equipment not only remains in service 50 years later, but also that it continues to look brand new. What is all the more astonishing is that the Rock Island railroad that built and owned the tower for its first 25 years was constantly going bankrupt.

Here we have selectors 7-11 and some better views of the glass encased relays. I am wondering where Metra obtains spare parts for such an odd duck. Perhaps due to the nature of the importation, the Rock Island was smart enough to lay in a large supply of spares from STC or perhaps when electro-mechanical telephone switching was on its way out someone at the Rock Island or Metra were able to lay in a supply from BT.

The Sequence Switch interlocking works through the rotation of the cams in response to point and route settings set on the control panel.  Depending on the cam position a series of electric contacts would either engage or disengage.  If the correct electrical path was present then a route would be locked and the signal displayed  At the time of its development it provided advanced NX capabilities with substantial savings in relay and wiring costs, however it never caught on due to it being offered by a telephone equipment company outside the usual signaling OEMs.

For those of you who can understand such things here are some regrettably low res scans of some circuit and interlocking diagrams for a sample STC SSI interlocking designed for British operation.

Layout and Interlocking, typical circuits

Circuit conventions and typical cam cutting

Sequence Switch positioning circuit

Sequence Switch restoring circuit

Sequence Switch lock out circuit

Typical point control and indication circuit

I'll wrap things up with a shot of the builder's plate so you can have your Moment of British Pride.

Well that's all I have.  It's a shame that this tower had to finally meet its end, but we should all be amazed that it lasted so far into the 21st century.  Let's hope that all of this unique equipment made it into some form of preservation instead of a dumpster or the scrapyard.


  1. "Slow line" = "Suburban Line" or "Dummy Line" as "Dummy" was Rock Island slang for the commuter trains to suburbia.
    "Fast Line" = "Main Line."

    There was a nice article about Gresham in the August, 1953, issue of Railway Signaling and Communications.

    Tom Wargin
    extra board towerman at Gresham, 1960-1963

  2. Might I respectfully ask for the source of the info that the panels at Tollerton and Pilmoor in the UK were also sequence switch interlockings? The two at Doncaster certainly were; I suspect that Tollerton and Pilmoor were conventional relay interlockings, although with control panels that strongly resembled Gresham and Doncaster. Many thanks and best wishes! Neil

    1. Probably a reply when I posted this on the forum. I don't know about those places to make it up XD

  3. No problem, that would explain it. To cross-check I even got a copy of the ST&C history (called Power of Speech A History of Standard Telephones & Cables 1883-1983) but that was 99% telephones so no help. Some old timers who remembered the Tollerton installation were pretty sure there had been no sequence switches. But, together with Pilmoor, it WAS ST&C. I will follow up a couple of leads at the National Rly Museum in York.

    1. Let me know what you find and I'll update the post as necessary.