Actually, train systems having its own assigned branches of police is common enough that there's a Wikipedia article[1]. Unique part is that the US doesn't have an umbrella national or state agency that such branches would be part of.

1: https://en.wikipedia.org/wiki/Railroad_police

BART exists in 5 counties (San Francisco, San Mateo, Alameda, Contra Costa, and Santa Clara) and multiple cities (SF, Oakland, San Jose, and multiple smaller ones).

Having people who have jurisdiction in any bart station is useful. And for example, consistent foot patrols are valuable to BART but not necessarily valuable to the city of Hayward, or whatever. As a concrete example, BART has more or less a goal to be able to put an officer on the car within a stop or two if you report on the app. That's logistically challenging for like a half dozen reasons if the bart police wasn't it's own org.

That's a good question, any I have no idea of the answer, but the port authority of NY and NJ has it's own police. I always figured it was because it spanned jurisdictions, but perhaps there are other reasons

Presumably because police usually only have jurisdiction within their employer’s city limits, and sheriffs’ employees only have jurisdiction within their counties, so if mass transit moves between multiple jurisdictions, then their police can have jurisdiction wherever the mass transit goes (even across state lines, because a lot of metropolitan regions served by the same mass transit system span two or more states).

No, there are a lot of jurisdictional issues and motivational issues.

An example of how it goes wrong is NYC. NYPD absorbed the transit police years ago and the role basically degraded to an overtime sink.

Also, with normal railways rails generate property taxes and towns often make deals where slivers of tracks get annexed to nearby towns or cities.

It's pretty common around the country. If it isn't a separate entity you end up having a department of the normal local police doing it separately anyway. Also BART goes through several different police jurisdictions and trains... move. I can imagine the mess and disagreements about which police department was responsible for answering a call or riding in a car as it moved between cities/counties.

It's a chain of command (i.e. BART police report to BART, Campus police report to the school, way less messy than BART or the school having to convince the jurisdiction how to police the area) and prioritization thing. Bespoke police agencies exist to police their niche to an extent that would be unjustifiable if it were coming your normal police department and taking resources away from their other tasks.

Transit police, campus police, DEA, ATF, etc, etc, etc. Their entire job is to harass people that the equivalent generalist police couldn't justify allocating resources toward.

Irate homeless? SFPD won't show, but BART Police will.

Drunk college kids shenanigans? Normal cops don't care, but campus police does.

Weirdos making machine guns or huge amounts of LSD? FBI don't care as long as you're not trafficking, they got real crime with real victims to chase. ATF and DEA care though.

Wash rinse repeat this pattern for literally every bespoke police agency.

Yeah, jurisdiction can be a theoretical reason for these agencies but once again it's still a priority thing. When agencies care jurisdiction and coordination isn't a problem. Having one agency span multiple only helps if you're chasing stuff so petty it would get dropped.