4 ms·

It must be one of those things that's harder than it looks - China had a train crash because they decided to roll their own signals.

exabrial 9y ago
Agree, it's obviously much different than a lighting effect in practice. What I'm trying reconcile is the hardware and software. Are there truly that many differences physically? The cost of signals probably comes from certifications I'm guessing
- Symbiote 9y ago
  Did you take any computer science courses on software verification? That is, proving your program does what you intend. It's complicated, difficult, and the people who can do this are expensive. Railway signaling systems, alongside air and spacecraft, and sometimes consumer devices that would be expensive to firmware update during the warranty, are the few places software verification is used. The specification we are verifying the software against must also be proven to be correct, including failing safe. The hardware also has to perform, at all reasonable temperatures, humidities, with vibration, poor power, and for an expected life of at least 50 years. The people installing the hardware must be checked, to ensure the work is done correctly. A crossed wire is a critical fault, I imagine there's a long and detailed procedure for connecting every wire. [1] is an interesting read, a retrospective on a bad crash in 1988 in Britain which led to many improvements in signaling safety. https://www.railengineer.uk/2014/01/06/25-years-after-clapham-a-review-of-signalling-procedures/ https://www.railengineer.uk/2014/01/06/25-years-after-clapha...
  - icebraining 9y ago
    But most of that work can be reused, you're not building the whole software from scratch each time, so those fixed costs get diluted. According to Wikipedia[1], there are about half-dozen commercial systems that get heavily reused. [1] https://en.wikipedia.org/wiki/Communications-based_train_control#Architecture https://en.wikipedia.org/wiki/Communications-based_train_con...
    - concerned_user 9y ago
      Unfortunately its not, yes framework is reused but business logic part is written from scratch most of the time and it is that part where most errors usually occur. That is why you can't just install SAP or Dynamics you need to program your logic and workflows into if first, the fact that framework itself is reused saves some time sure, but error probability increases with the amount of customization framework provides.
      - icebraining 9y ago
        I'm talking about software designed for railway signaling, like SelTrac, not generic business platforms like SAP.
        crocal 9y ago
        Seltrac will not be accepted by MTA. They have their own CBTC specifications and Thales (seltrac supplier) is engaged in a large program to create a compliant product.
        Symbiote 9y ago
        I don't know, but presumably there's still a lot of work - planning and then (re)locating points/switches - planning and then (re)locating signals or balises - updating trains - configuring the software to reflect the track and signals - verifying the final configuration
- pjc50 9y ago
  The signalling system probably has an extremely small amount of software in it, and if it's old the logic will be electromechanical; if a train is in the block, the entry signal will be red, because a particular relay is closed. The signal lamp itself is probably a small part of the cost compared to all the wiring and the time spent installing it. Edit: googling for "cost of railway signals" produces this very comprehensive PDF on the subject: www.credo-group.com/downloads/tco_of_rail_signalling_systems.pdf
- jon-wood 9y ago
  If you just look at the signals as "a little red light that tells the driver to stop", then its not that complex. However in the rail world signals encompasses much more than that red light. There's sensors on the track which detect when a train enters a section of the line and when it leaves, and on modern systems there's communication with the train itself to force the brakes on if the driver runs a red light, making it near impossible to enter a section with a train already in it. All those parts need to communicate with each other, and they need to be ridiculously reliable, and capable of self-reporting when they fail, because in the worst case a failure in any one of those components results in a few hundred people dying in a dark underground tunnel.

exabrial 9y ago

Agree, it's obviously much different than a lighting effect in practice. What I'm trying reconcile is the hardware and software. Are there truly that many differences physically? The cost of signals probably comes from certifications I'm guessing

Symbiote 9y ago
Did you take any computer science courses on software verification? That is, proving your program does what you intend. It's complicated, difficult, and the people who can do this are expensive. Railway signaling systems, alongside air and spacecraft, and sometimes consumer devices that would be expensive to firmware update during the warranty, are the few places software verification is used. The specification we are verifying the software against must also be proven to be correct, including failing safe. The hardware also has to perform, at all reasonable temperatures, humidities, with vibration, poor power, and for an expected life of at least 50 years. The people installing the hardware must be checked, to ensure the work is done correctly. A crossed wire is a critical fault, I imagine there's a long and detailed procedure for connecting every wire. [1] is an interesting read, a retrospective on a bad crash in 1988 in Britain which led to many improvements in signaling safety. https://www.railengineer.uk/2014/01/06/25-years-after-clapham-a-review-of-signalling-procedures/ https://www.railengineer.uk/2014/01/06/25-years-after-clapha...
- icebraining 9y ago
  But most of that work can be reused, you're not building the whole software from scratch each time, so those fixed costs get diluted. According to Wikipedia[1], there are about half-dozen commercial systems that get heavily reused. [1] https://en.wikipedia.org/wiki/Communications-based_train_control#Architecture https://en.wikipedia.org/wiki/Communications-based_train_con...
  - concerned_user 9y ago
    Unfortunately its not, yes framework is reused but business logic part is written from scratch most of the time and it is that part where most errors usually occur. That is why you can't just install SAP or Dynamics you need to program your logic and workflows into if first, the fact that framework itself is reused saves some time sure, but error probability increases with the amount of customization framework provides.
    - icebraining 9y ago
      I'm talking about software designed for railway signaling, like SelTrac, not generic business platforms like SAP.
      - crocal 9y ago
        Seltrac will not be accepted by MTA. They have their own CBTC specifications and Thales (seltrac supplier) is engaged in a large program to create a compliant product.
      - Symbiote 9y ago
        I don't know, but presumably there's still a lot of work - planning and then (re)locating points/switches - planning and then (re)locating signals or balises - updating trains - configuring the software to reflect the track and signals - verifying the final configuration
pjc50 9y ago
The signalling system probably has an extremely small amount of software in it, and if it's old the logic will be electromechanical; if a train is in the block, the entry signal will be red, because a particular relay is closed. The signal lamp itself is probably a small part of the cost compared to all the wiring and the time spent installing it. Edit: googling for "cost of railway signals" produces this very comprehensive PDF on the subject: www.credo-group.com/downloads/tco_of_rail_signalling_systems.pdf
jon-wood 9y ago
If you just look at the signals as "a little red light that tells the driver to stop", then its not that complex. However in the rail world signals encompasses much more than that red light. There's sensors on the track which detect when a train enters a section of the line and when it leaves, and on modern systems there's communication with the train itself to force the brakes on if the driver runs a red light, making it near impossible to enter a section with a train already in it. All those parts need to communicate with each other, and they need to be ridiculously reliable, and capable of self-reporting when they fail, because in the worst case a failure in any one of those components results in a few hundred people dying in a dark underground tunnel.