When it comes to transit, there is no 'mode'.

BRT makes a mockery of the 'best for' typology, largely because BRT is such a heterogenous category - quality varies from being heavy-rail competitive to being a bus with slightly nicer stops. (FTA labels aside, most 'BRT' isn't actually 'Rapid'--it lacks exclusive guideway).

American light rail hardly a 'clean' category either - tram-train is the norm, resulting in things like poor Portland (and formerly LA), where a through-corridor suffers a tram section in the middle. All are 'light rail' because they can't share track with 'heavy' rail - metros and commuter rail roads. Light rail is a vehicle crashworthiness standard. 

Having tried to identify all light rail stations (prior to the Transit Politic getting it done) actually transit is bafflingly heterogenous. Even with the 'New Starts' light rail systems, there are three distinct tropes: 1) tram-trains, 2) diesel, and 3) streetcars. 

1) German tram-trains (stadtbahn) are the trope definer for most American light rail - rapid transit service in the suburbs by using a former freight rail corridor, street-running in the central city (to avoid tunneling costs)*. In America, when we lack a freight rail corridor, we've built in freeway medians. 

2) New Jersey River Rail line is the trope definer, but Texas has built a lot of it. Save yourself the costs of electrification (and unsightly catenary) by keeping power generation onboard the vehicle. But that same fix makes stopping and starting both slower and more expensive, so stop spacing is much wider than for electrified traction**. 

3) Portland Streetcar is the trope definer for American streetcars. Portland's modern streetcar uses standard European trams, but that wasn't what got America excited about streetcars--rather, it was Portland's historic streetcars that were the original 'Development-oriented transit'. Supported by a local option tax along the corridor (American transit owes a huge debt to Powell's city of books). The whole concept of a 'pedestrian extender' and using a streetcar as a catalyst for real estate development in a proximate but inaccessible area (Pearl district). All of it street running from go. Over time, Portland has taken streetcar to the next tier, with a streetcar-only bridge, applying the same logic to places across a river. 

*If you put your city center light rail in a tunnel, so it's all rapid transit, it's evolved into a light metro.

** Battery technology may be a game-changer for diesel light rail, permitting both low-cost installation and electric traction levels of acceleration. Getting the brakes right is going to be the sticking point - batteries weigh a lot, so regenerate breaking practically necessary, and the 'trucks' (wheels) on rail vehicles are already complex and putting that much potentially explosive silicon near track is a bit perilous. 

Doubtful battery will be as relevant for trams/streetcars - anyplace that already has transmission lines will keep extending them, so the use-case is marginal--places without systems, for which the competition is BRT, which has a far lower initial investment cost and far better capability for incremental upgrades. 

Grades of Guideway

Right of way comes in three grades: 'Separated', 'Exclusive', 'Dedicated'.

Dedicated is the lowest grade. A lane nominally belongs to one mode, an obligation more commonly honored in the breach. Think of HOV lanes and bicycle lanes.


Exclusive can be though of as 'excluding'; exclusive use is maintained through barriers. Exclusive guideway is never continuous--there are always gaps in it, typically at intersections.

Separated is similar to exclusive, except that it is continuous. Typically, this requires grade separation, if only where the guideway intercepts other rights of way.


Heavy railways (commuter rail, freight rail) have separated guideway, for the simple reason that trains can't stop quickly. Where they must cross other roads 'time-separation' (railroad gates) are used. Where this generates unacceptable levels of delay for the cross-streets (a train can take minutes to pass) grade-separation is used. Typically, this requires an overpass for cars traveling over the road. In cases where there are too many roads, the railroad may be buried. The 'cut and cover' model of a trench with a roof were how the first subways were built. (Actual tunnels came later). In some cases (Alameda Corridor, in LA), digging an uncovered trench for the railway is simpler than many bridges.


Light railways have a mix of Separated, Exclusive, and Dedicated. Most light railways make use of some portion of old freight track, which are already time-separated. Some light rail systems provide Separated guideway by running down the center of freeways (Portland) or on elevated structures (Los Angeles, Salt Lake). Light rail on streets (a train in in tram-mode) typically has Exclusive guide way, with intermittent sections of Dedicated guide way. San Diego and Buffalo, two or the first cities to build light rail, made use of this. Light rail vehicles weigh less than heavy rail vehicles, and so can stop more rapidly. Streetcar/Tram vehicles weight even less, so they can stop faster and are even more suitable for on-street use.

The decision to use either Exclusive or Dedicated right of way is one of cost and safety. Without barrier separation, conflicts with turning cars results in accidents. Sadly, even Exclusive guide way is no guarantee against human stupidity.





Higher barriers, which prevent drivers from crossing them, may be safer. The trade-off is that first responders/emergency vehicles lose the ability to make left-turns and U-turns across the street, due to the curb. To accomodate this, curb heights vary between systems.





There is a fourth grade of right of way: Shared. This is what regular buses have. It offers no speed advantage to the transit vehicle over a regular automobile. Once the time consumed with exiting the flow of traffic, boarding and alighting passengers, and re-entering the flow of traffic is considered, transit vehicles in shared right of way travel at about half the speed of an automobile.


In contexts where right of way is scarce or political will lacking, Shared and Dedicated guide way are more common, to the detriment of transit vehicle speed, reliability and safety.  A streetcar operating in shared guide way has minimal advantage over a bus. (The minimal advantage  is that is cannot leave the flow of traffic, and so loses less time entering/leaving the traffic lane).



BRT systems are cheap to build, and largely unregulated. Consequently, they vary wildly. They make use of every kind of guide way.

Grade-Separated 




Time-Separated 
(LA Orange Line), for a time. Drivers ignored them, causing crashes. It still experiences issues with drivers running red lights. Grade-separation is in the works.

Exclusive - Fenced


Exclusive - Curb




Dedicated - Median



Dedicated - Side

A Concise History of Rail Transit in America

Modern transit comes from two historic lineages: Steam railroads, and horse-cart street railways. To operate in an urban environment, steam railroads underwent a number of modifications. To reduce conflicts with street level traffic, were either elevated or under-grounded. At some point, they were also electrified, typically use a third-rail system. Once that occurred, expansion required the continued use of grade separated, exclusive guideway, so no one touched the third rail and died. In contrast, street railways were electrified as trolleys, using a pantograph. (Cable-cars can be thought of as a 'dead branch' alternative to electrification). The converging modes of electrified heavy rail and street railways were hybridized as the "Inter-urban". Electrified the whole-way, using a pantograph, and running in a mix of at-grade and tunnels. After the second World War, almost all pure street running 'trolley' systems were 'bus-tituted' out of existence, while some inter-urban systems survived. The survivors all had some off-street running-way, viz: RTA Streetcars, San Francisco cable car, MBTA Green Line & Ashmont–Mattapan High Speed Line, SEPTA Subway–Surface Lines: Suburban Trolley Lines & Girard Ave Trolley, RTA Rapid Transit: Blue and Green Lines, Newark Light Rail, Muni Metro. Between ~1930-1972 is sort of a 'Dark Age' for urban rail--almost nothing new is built. Then there is a resurgence of heavy-rail systems to deal with traffic congestion: BART (1972), Washington Metro (1976), MARTA (1979), Baltimore Subway (1983), and Miami-Dade (1984). All run at-grade in the suburbs, and in tunnels in the city center. About 1980, America adopts the Stadtbahn/'City Rail' concept from Germany, and APTA coins it 'Light Rail'. It runs at-grade in the suburbs, and at-grade in the city-center, like the inter-urbans. Being regulated as 'light' rail, it is allowed to operate in mixed-traffic with cars, making it easier/cheaper to build. Over time, the surviving inter-urbans are rebuilt/revitalized, making use of the same vehicles as the new 'Light Rail' systems. Circa 2001, Portland reinvents the 'streetcar', which runs at-grade, in mixed traffic, with smaller vehicles, and making extensive use of single-track.

Now, to get back to what is 'Rapid' transit: Rapid transit is something that has it's own (unshared) guideway. Subways, elevated rail, commuter rail all clearly meet this standard, as do most of the 'Metro' systems of the 1970's heavy-rail revival. But the surviving inter-urbans and new light rail systems are a confusing mix: They have portions of exclusive guideway, so they have rapid transit portions. But LRT means 'Light Rail Transit' rather than 'Light RAPID Transit'. This gets confusion in the context of BRT, which actually means 'Bus RAPID Transit'. BRT gets developed in Latin America as a sort of bus version of a heavy rail system--buses with unshared guideway. But that's another topic. In summary: HeavyRail = Rapid, Streetcar !=Rapid, LRT !=Rapid...but does have sections that could be. (Cable-cars get lumped in with LRT largely on the basis of Cable-car != heavy-rail.)

And finally: Metro!=Heavy-rail, but Metro ⊂ Heavy-rail. Freight, Metro, Subway, Elevated, Commuter Rail ⊂ Heavy-rail.

=     Equal to
!=    Not equal to
⊂    Is a subset of.