How does a train know where it is?

[roysmith]

On the new M8 Metro North cars, the automated announcement system often gets confused. We'll pull into New Rochelle, and the thing will be saying we're at Larchmont, or whatever. In the age of GPS being able to tell me where I am to within a few feet, it doesn't seem like it should be very difficult to know which town the train is in. What's the story there?

[RearOfSignal]

ASI uses GPS for location information. While there have been problems related to GPS outages in the past, most problems are due to the ASI system simply freezing and not catching up properly to itself. Cues such as doors opening and train movement, signal certain actions by the ASI system. Sometimes if these actions happen in quick succession or out of order the ASI system freezes up and can make the wrong announcement.

[cobra30689]

On the Nippon Sharyo cars that VRE runs, the auto announcements are synchronized by an axle pickup sensor in the cab car. From what I understand back in the beginning a train was run on both lines, with a tech monitoring wheel rotations between stops, and that rotation count was stored in memory. Actual door opening at the station tells the system that the train has arrived, then it works its voodoo from there. It does have its hiccups now and then (especially when there are multiple cab cars in the consist), but for the most part it works pretty well.

[DutchRailnut]

with wheel wear that would be terrible inaccurate.
on MN cars it works in correlation between Conductor input (stations/vs skipped stations) and GPS .

[Literalman]

When I was commuting on VRE (1997-2017) I often heard wrong station announcements. Some (such as announcing the station we'd just left) might have been owing to being out of calibration, but others (such as leaving Fredericksburg and saying that the next stop was Union Station, Washington, DC) were hugely inaccurate. Also, many times the computer advised us to transfer to Metro at the next station owing do delays ahead on VRE. The alert was never true, in my experience, and often the next station was 20 or 30 miles from Metro. I've also heard wrong announcements on NJ Transit, both for the wrong station and for a station that the train didn't stop at.

[pnaw10]

Maybe this is a long shot, but would the eventual installation of PTC also provide a means for automated announcements to "know" a train's location without the use of GPS?

I'm just trying to think of how certain subway routes have automated announcements... GPS obviously isn't available when deep underground, so there must be some other way for "This is a Bronx-bound... six... local... train" to be accurately followed by "The next stop is ... Grand Central."

Unless subway conductors are pressing a button to manually fire every single one of these announcements, there must be some kind of transponders or sensors on or near the tracks.

[RearOfSignal]

ASI works fine for now, and MNR does not want to screw around any more with PTC than is already necessary.

[bulk88]

I'm just trying to think of how certain subway routes have automated announcements... GPS obviously isn't available when deep underground, so there must be some other way for "This is a Bronx-bound... six... local... train" to be accurately followed by "The next stop is ... Grand Central."

Unless subway conductors are pressing a button to manually fire every single one of these announcements, there must be some kind of transponders or sensors on or near the tracks.

Axle counter+door open/close button to recalibrate the axle counter. If a subway train goes unexpected express without the route changed entered the some of the annoucements will fire automatically. The axle counts between stations are stored in software. Time is not used to play annoucements. Due to wheel diameter problems on different cars, axle count is not 100% perfect and the counter is reset to zero at the next door cycle.