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I noticed a very specific pattern on passenger trains in the USA.

When they enter stations, they ring a bell repeatedly (roughly 1 hit per second) and their left & right headlights blink alternately.

Even very new rolling stock has this feature, such as the new Caltrain electric trains that were just put in service, build after a variation of an originally European design. Is it because of a local law?

This only happens on long distance & commuter rail, though. Never seen this in subways, for example.

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    To alert people that they're coming and to stay off the tracks. In my experience the bell rings the entire journey, not just when entering a station
    – Midavalo
    Commented Aug 14 at 20:24
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    Some waitees might be deaf (flashing lights) and some may be blind (repeated bells). Unlike an automoblile, the train can't easily stop. "Even very new rolling stock has this feature". Why should that make a difference to regulations?
    – Weather Vane
    Commented Aug 14 at 20:40
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    @littleadv there is a massive difference in their stopping ability. Automobiles are designed to stop very quickly.
    – Weather Vane
    Commented Aug 14 at 21:14
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    @WeatherVane it's just a matter of laws of physics - speed, weight, friction. Automobiles can stop faster than trains because they're smaller, often slower, and have rubber tires over asphalt and not steel wheels over steel bars. But other than physic the main difference is the ability to rapidly change direction which trains just don't possess at all.
    – littleadv
    Commented Aug 14 at 21:17
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    @littleadv The design of the train matters, namely the weight-to-braked wheel count ratio, the type of brakes, and line speed. Light rail and subway/metro trains stop far faster than mainline/heavy rail, even though they fundamentally run on the same wheels and track.
    – user71659
    Commented Aug 14 at 21:22

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Short answer: Yes, it's a regulatory requirement.

Longer answer: Rail transportation is tightly regulated everywhere in the world and, because of its very nature, the regulations are quite divergent from region to region (unlike road, air or sea transport). In the USA, the safety aspects of rail trasportation are regulated by the Federal Railroad Administration; and their regulations have influenced also Canada and Mexico. All peculiar features of North American trains, such as the characteristic horn sound, alternating headlights flashing when approaching a station or a level crossing (called "railroad crossing" or "grade crossing" in the USA), bells, etc., are regulated by the laws. Either the policymakers believed these were necessary for safety, or the railroad industry invented them and subsequently these features became fossilized in the regulations.

As a result, North American trains must sound their horns tons of times when approaching level crossings or stations at speed. When approaching slowly, they ring their bells and flash their headlights. In contrast, European trains sound their horns (very different sound) fewer times and (at least in countries I'm familiar with) only if the level crossing has no barriers and/or warning lights.

Subway trains, while also running on steel rails and thus very similar from the physical point of view, are a completely different universe from the operational and legal point of view. This is true not only in the USA, but in just about every country that has them. In France, for example, normal trains run on the left, while subways run on the right (except Lyon).

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  • In UK, trains have lineside signs which mandate the audible horn, siren, whistle etc., but AFAIK they don't have flashing lights. They also sound their horn at times to alert a signalman to their presence, or to indicate they have completed a manoeouvre. Much of this is now automated.
    – Weather Vane
    Commented Aug 15 at 19:53
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    A North American train only needs to sound its horn when approaching an "open" or "half-barrier" crossing. At a "full barrier" crossing (road completely obstructed by crossing gates), the horn isn't needed.
    – Mark
    Commented Aug 15 at 21:09
  • @Mark interesting! I'm yet to see a youtube video in which a North American train passes through a grade crossing silently :-)
    – Johnnyjanko
    Commented Aug 16 at 5:52
  • @WeatherVane - from about 1963 onwards British trains had a yellow rectangle of specified shade and minimum size on the front, to aid visibility, to increase safety for track workers as well as passengers. This became mandatory, and in the 1980s a further requirement for a headlight of specified type was added. In approximately 2010, the yellow panel part was relaxed provided two headlights of specified type were fitted. None of these white front lights are allowed to blink.
    – Michael Harvey
    Commented Aug 16 at 13:41
  • @WeatherVane - horn use is manual on UK trains. Other situations where a driver may use the horn include when they see track workers anywhere on or near the track (they are supposed go to, or be in, a place of safety and to raise one arm above their head).
    – Michael Harvey
    Commented Aug 16 at 14:12

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