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Intermediate Point Headway Control: Questions and Answers

Written for the TTC by George Davidson; Contributed by George Davidson

Originally Published in 2002

Q. What is the IPHC and what is it for?

A. IPHC or Intermediate Point Headway Control, is in the most basic of terms, a computerized version of the old mechanical ATD. Instead of a 35mm film going through a clock work, we have a computer and it's program. Current schedules are downloaded into the computer and all the dispatches from the terminals are controlled automatically by the computer and it's program. Also controlled by the same program are all the IPHC points along the line. These points regulate headways between trains determined by a written schedule.

Q. What was wrong with the old ATD? It seem to work pretty good and didn't cause as much trouble.

A. Well the old ATD was just that, old. The brass clock works that ran the system were over 35 years old and replacements would be very expensive for a technology that was very outdated. The move to the IPHC computers was an attempt to update our method of train control and dispatching to the existing technologies.

Q. Okay, so we modernize our systems. Does it really work and how is it going to benefit me and my customers?

Crowded Bus

A. To answer this question I'll have to ask you to think of the IPHC as an equalizer or referee. We all know that the equipment and the skills of the operators are not equal. You have very senior crews with a wealth of operating savvy and you have the junior crews who are well on their way to learning all the tricks but are not yet there. Now mix in a variety of equipment from the, older and tired H-1's to the newer and tighter H-5's and you can see the problem. Seasoned crew "A" leaves Finch in H-5's and 4 min. later junior crew "B" leaves Finch in H-1's with one ECO. Guess who gets hammered to the doors and is eating dust before they get to Eglinton Stn.? The IPHC does not allow the '5's to run away from the slower train and in turn the passengers don't end up being crammed into a train like the AM Finch bus. The faster train can only go as fast as the IPHC allows it to go. If the running time is 12 min. to Eglinton and the faster train does it in 9 min. then the IPHC won't allow the train to depart for 3 min. after arrival. When train "A" leaves Eglinton, Train "B" will be carrying the proper headway if the running time is maintained.

Q. Then if I operate as I usually do I get penalized and have to sit and wait at IPHC points because I'm quicker then the crew behind me. Why shouldn't I just trudge along enjoying the scenery?

A. The IPHC is not an automatic headway control device. It depends on the crews to maintain a reasonable headway and running time. The main purpose of the IPHC is to fine tune the headways to allow trains to pass a given point in a consistent time frame. Equalizing out the headways between the trains allows the most efficient distribution of passenger loads for all the trains on the line. Think of the IPHC as a tool to assist you in the daily operation of your train. You have a job to do and believe it or not the IPHC was developed to be a benefit to you and your passenger. Gone are the days of trains leaving the terminal to disappear from sight of the following train until the end of the line. Being in a gap constantly is both frustrating and stressful and shouldn't happen to anyone who is trying their best to do the job required.

Q. What then is the so called "voice activated signal" when I know that I'm early at an IPHC point and call in to see if the IPHC is working?

A. The tower Inspectors have the capability at Hillcrest, to override the pre-written schedule of the system, using the tower workstation's keyboard commands. They can "Train Dispatch" at a particular IPHC point, or at several points. They can pre-program negative or positive "off sets" to either hold or dispatch trains. They can also "Unscheduled" or "Disable" an IPHC point to allow trains to proceed through without being held. Examples being, North York Center, Islington and Yorkdale stations.

Q. If those stations are "off line" then why are we still be held there, especially on our run in and break trips?

A. The IPHC is only a device to dispatch trains and regulate the headways. It does not in anyway effect or override the main safety functions of the signal and block protection built into the subway's wayside signal system. What you may perceive as the IPHC, is actually the signal and block protection, functioning as it should. If trains are backed up into a terminal the wayside signal system will allow passage of trains only if the proper conditions and safe distances between trains are met. When trains are backed up leaving a terminal you may receive a white guard dispatch light from the IPHC but the departure wayside signal will still be double red! The previous departing train is still in the block ahead and the wayside signal system is working as it should.

Q. Does this explain why we get the white guard light and green signal shortly after arrival at the end terminal?

Release Switches

A. In part yes. The IPHC program unfortunately works effectively only when the line is on time or running ahead of schedule. When the line becomes late and trains back up the IPHC dispatches trains when they arrive. There is however approximately seventy seconds for the switches to release and the departure signal light to cycle green. The running times for the current schedules are now under review and soon we hope to have a more realistic running time incorporated into the IPHC program.

Q.Why are some of the mainline IPHC points on some days and off others?

A.The IPHC system is basically a tool for the Inspectors to use. Every tower Inspector has their own method of utilizing this tool to what they believe is the most effective method for the given circumstances. As you know, the subway lines rarely operate each and every day the same way. We are striving to establish a consistent method of operation, but this is still a new and not totally completed system.

Q.When we are deadheading in the morning or after service why does the IPHC still hold us?

A.The IPHC schedule is pre-written and the system cannot tell the difference between service trains and a deadhead. The planning department writes the schedule and puts all the trains on predetermined headways from point to point. It is up to the Tower Inspector and yourselves to communicate that you are running private and would like clearance through an IPHC point. Revisions are being made in the program to address this point.

Q. How can I help to make the system more efficient and provide a better service for my customers with the minimum of delays to them and myself?

A. One of the biggest enemies we all have is lack of knowledge. If you are unfamiliar with why the IPHC does what it does ask about it. Get to know what the positive and negative sides of the system are. You all know how to take advantage of the wayside signal system. Don't you? The IPHC is no different. Know your running times and ask for the departure times of the various IPHC points for your Run. Ask if there are any offsets on the line and what points are operational and what are off-line. Ask if there are holds in place. Especially if you hear of delays on the line.

The IPHC has a wealth of information we can relay to you. It will tell you to the tenth of a second the headway your carrying and the headway of the previous train. It can tell you in most cases if the light you saw cascade and trip you (at and approaching and leaving IPHC locations) did in fact cascade. All this and more information is available to all the crews. A detailed log is kept by the IPHC on disk for every event connected with it. Asking is all that is required to do. We will be happy to supply you most information such as headways immediately. Log information may require more time as it is separate process. The IPHC was designed to assist us all in providing a more reliable and efficient subway system. Becoming familiar with how and why it works can only benefit all of us in the long run.

Q. Just how reliable is the system, and should we still call after one minute on a double, even at an IPHC point?


A. The reliability of the system is improving on an ongoing basis. There are still bugs to be worked out and software glitches still show their ugly heads and baffle the engineers. Overall the system is working much better now then it was a year ago and will be much better once the new schedules and running times are incorporated into the IPHC programs. If you are at a double red for more then one minute, always call in to verify if there is a hold at that location, or if the IPHC has dropped a dispatch. It is very hard to monitor each and every point constantly. There still are some very strange bugs in the system, that are at times, very frustrating for us to figure out what it is trying to do.

Q. We have heard that the IPHC can tell if your train has tripped a signal anywhere in the system. Is this true?

A. The IPHC can record the passage of a train through an interlocking section of signals, but can only tell if a specific signal which is controlled by the IPHC has been tripped (after a very specific sequence of events occur) or if there has been a malfunction which caused a train to trip that signal. All terminal signals i.e. X-10, X-12, X-16 etc. that protect the crossovers are on line and are recorded. Added to this system is a new event recorder system that is connected to the majority of the signals. This new event recorder is the property of the Signal department and is now being installed in the Hillcrest Control Center to replaced the obsolete pen recorder. This system along with the IPHC log will record all signal malfunctions and irregularities that may occur. Train crews should now be more confident in knowing that signal malfunctions causing signal violations will be detected as just that. A signal malfunction. It will be in your best interest to report all signal occurrences so the true cause of the occurrence can be determined.

I hope that the information enclosed has shed some light on the how's and why's of the IPHC. If you have any questions either general or technical please give us a call at Hillcrest Tower and time permitting we will answer your questions to the best of our abilities either in person or by internal mail.


It will Probably Hold us Contraption


* Intermediate Point Headway Control