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You can go down a very deep rabbit hole learning about the history of train signaling. Trains and subways have had centralized signaling for…I’d have to look it

by wrs 1y ago

You can go down a very deep rabbit hole learning about the history of train signaling. Trains and subways have had centralized signaling for…I’d have to look it up, but 100 years surely? It’s the only way to safely have more than one train running at a time (i.e., sharing the track) with a dense schedule. The “local first” procedure when it fails is to radically reduce service and slow down the trains. Wikipedia has a good survey [0]. [0] https://en.wikipedia.org/wiki/Railway_signalling https://en.wikipedia.org/wiki/Railway_signalling

jsmith45 1y ago
Block based automated signaling can technically be implemented as a primarily local system. Each block needs to know if there is a train in itself block (in which case all block entrance signals must show stop, and approach signals indicate that they can be entered, but the train must be slowing, so it can come to a stop by the block entrance signal). It must also know about a few preceeding blocks for each path leading into it, so as to know which contain trains that might be trying to enter this block, so it can select at most one to be given the proceed signal, and others to be told to brake to stop in time for the entrance signal. While it is nice if it knows the intended routes of each train so it can favor giving the proceed indicator to a train that actually wants to enter it, but if it lacks that information, then giving the indication to a train that will end up using points to take a different path doesn't hurt safety, just efficiency. Of course, centralized signaling is better, allowing for greater efficiency, helps dispatch keep track better track of the trains, makes handling malfunctioning signals a lot safer, among many other benefits. But it doesn't mean local signaling can't be done.
- wrs 1y ago
  Yes, block based signaling is what I interpreted “local first” to mean in this context. It works, but it slows everything way down. I don’t know, but I would imagine, there’s still a block based setup as a failsafe backup in most or all modern rail systems.
stickfigure 1y ago
Yeah, we literally invented positive train control because trains crashed too often.
reaperducer 1y ago
The New York Times had a very visually compelling article a few months ago about how a good part of the city's subway system is still manually-operated. https://www.nytimes.com/interactive/2025/04/20/nyregion/nyc-subway-signals.html https://www.nytimes.com/interactive/2025/04/20/nyregion/nyc-... For me, this was the best picture: https://static01.nytimes.com/newsgraphics/2025-03-10-subway-signals/_images/oldsignals_scroll-@@-2400@x2.webp https://static01.nytimes.com/newsgraphics/2025-03-10-subway-... Someone has to stand at that machine 24 hours a day and push and pull levers to keep the trains from whacking one another.

by wrs 1y ago

jsmith45 1y ago

Block based automated signaling can technically be implemented as a primarily local system. Each block needs to know if there is a train in itself block (in which case all block entrance signals must show stop, and approach signals indicate that they can be entered, but the train must be slowing, so it can come to a stop by the block entrance signal). It must also know about a few preceeding blocks for each path leading into it, so as to know which contain trains that might be trying to enter this block, so it can select at most one to be given the proceed signal, and others to be told to brake to stop in time for the entrance signal. While it is nice if it knows the intended routes of each train so it can favor giving the proceed indicator to a train that actually wants to enter it, but if it lacks that information, then giving the indication to a train that will end up using points to take a different path doesn't hurt safety, just efficiency. Of course, centralized signaling is better, allowing for greater efficiency, helps dispatch keep track better track of the trains, makes handling malfunctioning signals a lot safer, among many other benefits. But it doesn't mean local signaling can't be done.

wrs 1y ago
Yes, block based signaling is what I interpreted “local first” to mean in this context. It works, but it slows everything way down. I don’t know, but I would imagine, there’s still a block based setup as a failsafe backup in most or all modern rail systems.

stickfigure 1y ago

Yeah, we literally invented positive train control because trains crashed too often.

reaperducer 1y ago

The New York Times had a very visually compelling article a few months ago about how a good part of the city's subway system is still manually-operated. https://www.nytimes.com/interactive/2025/04/20/nyregion/nyc-subway-signals.html https://www.nytimes.com/interactive/2025/04/20/nyregion/nyc-... For me, this was the best picture: https://static01.nytimes.com/newsgraphics/2025-03-10-subway-signals/_images/oldsignals_scroll-@@-2400@x2.webp https://static01.nytimes.com/newsgraphics/2025-03-10-subway-... Someone has to stand at that machine 24 hours a day and push and pull levers to keep the trains from whacking one another.