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Related: There was that story about how axle count on Swiss trains can't be a multiple of 256 (2^8) [0], and a comment mentioned that German has a limit of 252

by SilasX 6y ago

Related: There was that story about how axle count on Swiss trains can't be a multiple of 256 (2^8) [0], and a comment mentioned that German has a limit of 252 "just to be safe": https://news.ycombinator.com/item?id=12071686 https://news.ycombinator.com/item?id=12071686 [0] Because there are only 8 bits to store the axle counter for whether the train has passed, and you don't want overflow to zero.

jbay808 6y ago
That's interesting! But a safety factor does make sense here if you're worried about a small probability of double-counting an axle.
- Doxin 6y ago
  Double counting an axle is a critical failure for an axle counter. It could keep a track section blocked until someone comes to fix the axle counter. Even worse it could unblock a track section while there's still (part of) a train on it. If you're worried about double-counting an axle you probably need to design your axle counters better.
cleansingfire 6y ago
Sounds like Swiss trains need a single axle connector to ensure all trains have an odd number of axles.
- skykooler 6y ago
  Or use seven axle locomotives. (Did a bit of research, at least one has existed: http://cs.trains.com/trn/f/111/t/72627.aspx http://cs.trains.com/trn/f/111/t/72627.aspx)
  - jcranmer 6y ago
    Broadly speaking, with electric drive, it generally makes the most sense to have both your four and aft bogies as all-drive axles, and there's no real compelling reason to have those axles be different. In practice, this means that the vast, vast majority of modern locomotives are B-B or C-C (2-axle/3-axle bogies, respectively), with B-B-B (3 2-axle bogies) probably the next most common. Other configurations are almost entirely experimental or one-off locomotive configurations. In the steam era, things were rather different. Steam power requires a linkage of rather large drive wheels, which generally means you need unpowered leading and trailing wheelsets for stability and performance purposes. The trailing wheels are almost purely to support the weight on the back end of the locomotive, but leading wheels also help with curves in addition to supporting weight on the front end. Single leading axles were quite common in the steam era, with 2-8-0 being perhaps the single most common arrangement.
sedatk 6y ago
They didn't even trust 255, 254 or even 253. It had to be 252.
- deepsun 6y ago
  Well, train carts usually have 4 axles.
MikeTheGreat 6y ago
I wonder how they arrived at that number? It seems like you'd want to do something like: (max axles) = 256 - (% of missing an axle) * (number of axles) More or less. (Obvs you'd have to know the chance of missing an axle - but without a reasonable guess at this it seems you couldn't really set the max value accurately)

by SilasX 6y ago

jbay808 6y ago

That's interesting! But a safety factor does make sense here if you're worried about a small probability of double-counting an axle.

Doxin 6y ago
Double counting an axle is a critical failure for an axle counter. It could keep a track section blocked until someone comes to fix the axle counter. Even worse it could unblock a track section while there's still (part of) a train on it. If you're worried about double-counting an axle you probably need to design your axle counters better.

cleansingfire 6y ago

Sounds like Swiss trains need a single axle connector to ensure all trains have an odd number of axles.

skykooler 6y ago
Or use seven axle locomotives. (Did a bit of research, at least one has existed: http://cs.trains.com/trn/f/111/t/72627.aspx http://cs.trains.com/trn/f/111/t/72627.aspx)
- jcranmer 6y ago
  Broadly speaking, with electric drive, it generally makes the most sense to have both your four and aft bogies as all-drive axles, and there's no real compelling reason to have those axles be different. In practice, this means that the vast, vast majority of modern locomotives are B-B or C-C (2-axle/3-axle bogies, respectively), with B-B-B (3 2-axle bogies) probably the next most common. Other configurations are almost entirely experimental or one-off locomotive configurations. In the steam era, things were rather different. Steam power requires a linkage of rather large drive wheels, which generally means you need unpowered leading and trailing wheelsets for stability and performance purposes. The trailing wheels are almost purely to support the weight on the back end of the locomotive, but leading wheels also help with curves in addition to supporting weight on the front end. Single leading axles were quite common in the steam era, with 2-8-0 being perhaps the single most common arrangement.

sedatk 6y ago

They didn't even trust 255, 254 or even 253. It had to be 252.

deepsun 6y ago
Well, train carts usually have 4 axles.

MikeTheGreat 6y ago

I wonder how they arrived at that number? It seems like you'd want to do something like: (max axles) = 256 - (% of missing an axle) * (number of axles) More or less. (Obvs you'd have to know the chance of missing an axle - but without a reasonable guess at this it seems you couldn't really set the max value accurately)