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Wednesday, November 13, 2019

CBTC is a Scam and the MBTA Backs Me Up

Communications Based Train Control promises higher capacity at lower costs thanks to the magic of WIRELESS TECHNOLOGY!  However almost every real world situation that makes use of it seems to wind up costing a ridiculous amount, having major reliability problems or both.  It turns out that with signaling there is no free lunch, but when faced with overcrowded subway trains, planners can't help but get seduced by those lovely braking curved.  I mean fixed block, that's so 1890, surely we can do better!

In 2016 the MBTA was conducting a capacity study for the Red Line, which currently has a somewhat anemic throughput of 13tph in the peak period.  With new rolling stock on order, the (T) would literally have more cars than it could run.  The current signaling system made use of fix block audio frequency cab signals installed in the 1980's.  This is similar to many other transit systems such as the CTA, WMATA and BART.  Of course CTBC is the to go technology for capacity expansion and the study quickly confirmed this.  Oh wait, it didn't.

• A detailed analysis assuming a moving
block CBTC system on the Red Line was completed.

• Analysis found that a CBTC system would produce
an improvement of just one train per hour beyond
the improvement from the new cars and minor
system changes.

• Major Red Line capacity improvements can be
achieved without implementing very costly CBTC. 

• Long dwell times in the downtown area and close
spacing of stations limit CBTC as much as they
limit fixed block systems.

That's right, just like the costly NYC Subway L Train CBTC system only increased capacity by 2 trains per hour, applying CTBC to the Red Line would only improve capacity by 1 train per hour over a fixed block alternative.  Past a certain threshold capacity is limited by dwell time and the efficiency of terminal interlockings. The study also found...

• The shorter the block length, the closer the
system is to the ideal CBTC (moving block)
braking distance
• MBTA block lengths in the central subway already
average less than 500 feet (6 car trains are 416
feet long)

It's nice to see that for once a transit agency actually ran the numbers and decided that CBTC just wasn't worth it.  It turned out the best way to increase the capacity was simply to allow the new rolling stock to use updated braking curves that will result in later braking and more aggressive cab signal speed stepdowns.  Also the 1980's audio frequency cab signal system will have its components replaced with digital versions that have faster reaction time and thus allowances for less conservative block progression. 

6 comments:

  1. This comment has been removed by the author.

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  2. I agree with you, and it's expensive.

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  3. Really late to the party here, but CBTC as implemented by the MBTA hardly seems the best practice to examine. Between the agency's braking model, the RL's extremely high dwell times and what I'm sure were some highly conservative assumptions regarding CBTC rules design (are trains allowed to follow each other w/in station limits), this seems a sandbagged estimate. Systems in Europe/in YVR regularly run 34-40tph with CBTC, which no fixed block system outside Moscow has ever been able to replicate (Moscow does it with fanatical dwell time control and, I'm sure, some pretty generous signal design assumptions). Even the old, somewhat conservative the NYCT CBTC system installed on the L is capable of 30+ tph -- the L just has a crappy terminal at 8th Ave + power supply issues.

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    1. Whole whole point is that real world scenarios will always limit TPH to something within the realm of fixed block and of course there is the option of using fixed block with a potential vehicle borne system for close in running (think adaptive cruise control).

      Costs and reliability aside, as we move closer to the age of crippling cyber-attacks, safety critical systems need to be as decentralized and as fail safe as possible.

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    2. The L line is a bit of an outlier. Again, most CBTC installs are able to reliably achieve capacities well north of 30tph, ie real world allows full realization of the tech's potential. We just suck at rapid transit design/ops in North America. One thing we do wrong a lot is excessively overlay duplicative tech rather than fixing an underlying condition, which is why I'd suggest anyone considering investing in a blended system just go the full mile with moving block. You'd realize a good-sized ROI in maintenance/runtime savings.

      The cybersecurity issue is whose complexities I don't know enough about to be able to speak authoritatively on (though I know it's being studied/fixed extensively), but I generally make a point to not run counter to global best practice on these sorts of issues, which continuing investment in fixed blocks would constitute and then some.

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  4. The MTA seems to disagree and claims the L train is one of their best performers: https://new.mta.info/projects/cbtc

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