When you eliminate block signals and track circuits and use audio or radio-frequency distance sensing for movable blocks, you can increase the capacity of a constant-speed double-track railroad by 50%. That is one reason why Metro North has moved in that direction. The subway system is doing the same with but with the addition of automatic operation, now the L 14th Street Canarsie Line, soon the 7 Flushing Line, and then the common Bronx portion of the 2 and 5 for the first use of trains that run on conventional block signals with the old trip-hammer safety device and the movable block with automatic operation system. Redudancy is built into both the Metro-North and Subway systems, and the two channels in use have to agree for operation to continue. And manual override at restricted speed is, of course, possible. The subway does not use cab signals in addition, but I believe Metro North does, or has an indicator that provides the same information. The whole shebang requries less maintenance than wayside sighals.I believe Bay Area and Washington Metro do not or did not have the redundancy in their automatic operation that New York has.
I don't believe any one has "rolling blocks" yet. That is one of the things on the wishlist for eventual versions of PTC and the like.
Just because there are no lineside signals does not mean there are no blocks and the associated track circuitry. On the exCNW ATC lines where there was no wayside signals, there were still block boundries. They weren't marked but eventually you knew where they were. (The UP added a directional block system using only cab signals on a line or two in northeastern Kansas. From what I understand, they placed signs at the block boundries. The signs read something like, "Cab Signal Aspect Change Location." The signs may not be maintained anymore. They were needed when we were required to run a train length after the cab signal cleared up at other than a signal or known aspect change location. Like Zug, we can comply immediately when the cabs clear up.)
Block systems also provide broken rail protection. Some have invisioned going to a completely communication based control system that does away entirely with blocks and signals. Because of the broken rail protection, there will likely always be some form of signal circuitry. It may be removed from actual traffic control (except when a break in the circuit happens) and the blocks may be longer than conventional signal blocks, but I'd bet there will always be a track circuit on busy lines.
Jeff
I believe that the protection afforded by slide fence circuits is just as much a part of the system as that afforded by the broken rail detection.
Johnny
Some bridges also have fire protection circuits tied into the signal system.
jeffhergert Some bridges also have fire protection circuits tied into the signal system. Jeff
Don't overlook Slide Detector Fences.
Never too old to have a happy childhood!
I will check on your statement about nobody having rolling blocks. I was told in 1996 that is was in the works and assumed it had been implemented by now.
But I was also told it wouldl handle objects on the track and broken rails, since the rails are the carriers of the sensing high-frequency signals.
daveklepper I will check on your statement about nobody having rolling blocks. I was told in 1996 that is was in the works and assumed it had been implemented by now. But I was also told it wouldl handle objects on the track and broken rails, since the rails are the carriers of the sensing high-frequency signals.
In the works and in place are vastly different things. MN, like many other commuter railroads is having trouble getting fixed block PTC in place much less getting something up and running that no one else has on a mixed use railway.
Change is a process, never instant and never goes the way we expect it.
That being said, the trackmen's poor experience with TAWS and ARES what it was, I'm happy with a little more redundancy to help process what's going on around me.
Dave: read the article in Railway Age posted by BALT concerning how fixed blocks and wayside signals got retained unnecessarily by the rail operating departments in PTC adoption.
C&NW, CA&E, MILW, CGW and IC fan
mudchicken , I'm happy with a little more redundancy to help process what's going on around me.
, I'm happy with a little more redundancy to help process what's going on around me.
As a pilot must agree. Cannot list the number of times when one of the following ADF, consulan, VOR, Loran, OMEGA, INS, IRS, GPS, ILS failed to provide proper info but some of the others came to the rescue and make a trip a non event.
IMO too many system developers try to put all your eggs in one basket.
blue streak 1 mudchicken , I'm happy with a little more redundancy to help process what's going on around me. As a pilot must agree. Cannot list the number of times when one of the following ADF, consulan, VOR, Loran, OMEGA, INS, IRS, GPS, ILS failed to provide proper info but some of the others came to the rescue and make a trip a non event. IMO too many system developers try to put all your eggs in one basket.
and overlook the hole they designed in the basket!
Checking my source, you are right about MN and NYCTA. On both, fixed blocks remain. Possibly going to "rolling blocks" in the future. But properly designed, rolling blocks without track circuits can not only detect broken rails, but sun-kinks as well, something trackcircuits have not yet been developed to do!
Metro North has removed wayside signals from most of its third-rail-operated lines, except at interlockiings. The L line has removed them between 8th Avenue and Eastern-Parkway-Broadway-Junction-East-New-York. Between there and Canarsie, both ATO and the wayside signals with the old trip hamers remain operstional. I can understand this, because in the old days there was a Broadway Elevated - Williamsberg Bridge local Chambers Street - Atlantic Avenue service with alternate trains exdtended to Canarsie; these remained from the days before the completion of the 14th Street Canarsie line. The J, M,and Z routs are not ATO, and at times trains from these routs may require diversion to Canarsie. West (RR south!) of EP-BJ-ENY diversions from other routes would require back-up moves and are not likely, so only ATO-equipped trains operate, the L line.
My source says there may be one signalled North American light rail line that does use moving blocks and suggested I check on it.
Without a track circuit, I would like to know how the broken rail is detected. Besides the possible derailment, that is.
Jeff: Didn't the old Galena Division of the CNW get along fine without lineside signals for years, at least until the UP installed their style signals within the past ~5 years.
I should have said "conventional track circuit," and was wrong to leave out the word conventional. These are of two types. One is the normal dc or ac, with insulated joints between block. Either a short, from occupancy, or open, broken rail, means a stop indication, and approach at the preceding block. A more sophisticated block track circuit system does not have insulated joints, but a resonant specific frequency shunt between rails at the demarkation between blocks, and the track circuit is tuned to measure changes in the reactive and resistive impedences between shunts, with changes either way causing the stop signal. These are both what I would call conventional.
A moving block system sends signals from the train down both rails with respect to ground. Occupancy, or anything abnormal, will send back a reflected signal, with or course a break in the rail a very different signal than occupancy. Also, the reflected signal from both are compared.
The rolling blocks I've read about for freight railroads have all been on communication based detection of trains, mow equipment, switches, etc. Each piece reports it's location and/or status. No signal sent out through the rails. Although I recall hearing that (I think) Rockwell-Collins was working on a collision avoidance system along those lines years ago. I think they did some testing on the CRANDIC, since the person who told me this worked for them.
But the pure communications based systems will not detect a broken rail.
Possibly a practical eventual system would use both communications and track-wave-guide technologies to provide both redudancy and no reduction of safety from track-circuit fixed-block tedhnology.
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