Passenger Rail and urban rail

When I started out in transportation consulting, I couldn't understand why the commuter rail and light rail couldn't just share track. (Especially baffling in Utah, where the same track that ran TRAX by day ran freight rail by night). 

I still sometimes find the division between passenger rail and urban transit kind of quirky. As a transit user, commuter rail is just another form of transit, a commuter bus on rails--lousy frequency but higher speeds. But as an operator, those are two unlike things: one is a railroad, and one is not. Partially, it's regulatory: railroads fall under the FRA, urban transit falls under the FTA[2], and the two have different standards.  But it's also historical, a matter of convergent evolution. Railroads start with steam engines, urban transit with the horse drawn coaches. 

My memory has it in Europe that the regional rail systems don't so much end as they peter out. The further out stations get fewer trains per day. (Less active station inline, also-not every train stops at every station). Which can be nice: it's possible to develop passenger rail into commuter rail simply by increasing frequency. Ie: There is an Amtrak train in California with more daily trains than most commuter rail systems. And hence to build a regional rail system by increasing headway further and running it outside of peak hours. The NJ-NY PATH train has followed that path to quite a peak: 15m headway, including subway sections[1]. 

There isn't really a division (technologically speaking), between passenger, commuter, and regional rail systems, as far as right of way is concerned. There are certainly operational differences in the cars used: different journey lengths require different amenities. A seat acceptable for an hours ride isn't for 8 hours. And long-distance journeys require things like outlets, reclining seats, a bathroom, a dining car, etc.  

Part of the confusion comes from legacy metro systems: in an urban context, the only way to get a railroad through a street grid is to either put it on an elevated or in a subway [1] . So some of the first 'subways' are just steam trains run underground (which is why some NYC subways have steam grates). But at the same time, STREET RAILWAYS are also being put into tunnels, via a sort of convergent evolution. (Stage-coach --> horse-drawn omnibus --> trolley->subway). That streetcars could also (safely) travel much faster underground made their operations more railroad like, and in turn required more railroad-like geometric design. 

Muddying the waters are inter-urbans. The supply of railroads had been overbuilt, truck freight on highways was a growing competitor, and there were some under-used railways available for conversion into a sort of railway/trolley highbred, running as a railway outside of cities, and as a trolley within it. Functionally an early light rail. 

Further muddying the brew is BART and WMATA: A Tomorrow-Land reinvention of urban mass transit, with more widely spaced rails, operating as a railroad in the suburbs, and as a metro in the city center (underground with widely spaced stations). 

But their is a well-maintained regulatory distinction, in terms of vehicles: heavy rail and light rail, and it has to do with crash standards. Since passenger rail sometimes shares ROW with freight railroads, the cars have to be designed to withstand crashes with heavier vehicles. While 'light' rail vehicles are much lighter and less sturdy. ('Light' is relative--your standard light rail vehicle can still shred the heaviest passenger car like it was an aluminum can).

[1] Indeed the whole subway/overground distinction is kind of a humbug--trains run best on flat/level, and the ROW gets built to reflect that, regardless if the undulations of the terrain means that results in the ROW being underground or overground. Plenty of subways have overground and/or elevated sections. 

[2] Originally the UMTA - Urban Mass Transit Administration. Original name clearly established what it wasn't about: non-mass transit (taxis, jitneys) or rural transit (stages, coach buses).