Electricon's answer is right on...except, as he pointed out, when it's not, which is why it's such a tough question to answer.

(1) Light rail trains don't share lanes with other traffic while streetcars will... except that some light rail does operate in shared ROW.

(2) Light rail trains can use multiple units while streetcars don't....typically true, but many streetcar vehicles can be coupled into at least 2-units.

(3) Streetcars can turn tighter curves than light rail trains....can and should are not the same thing. Light rail typical is 82-feet, streetcars you can ratchet down to 50-feet... but then, light rail can theoretically go tighter. And most design specifications you'll see for streetcar systems call for the light rail standard anyways (less wear and tear on the vehicles; higher operating speeds). But this is an important factor in vehicle selection, so it's as good of a criterion to use as any.

So like anything else, the answer is, it depends. The light rail/streetcar distinction is very much an American thing, and as I think Cirrus pointed out, it stems from our habit of using light rail as a "metro light." But that is by no means necessary - plenty of places mix and mingle the two vehicle types operationally.

In the U.S., I think the easiest distinguishing characteristic is shared ROW versus not. That might require us to re-think some of the older light rail systems, but so what?

But, it is not yet under construction.

bunt_q, what about max speed? Could that be a distinction?

Not when the same transit agency is using the exactly same equipment to do both functions. Although generally streetcars are designed to go a max of 45 mph while light rail trains are usually a max of 55 mph. But the real max speed for streetcars in operation is the speed limit of all the other traffic on the same streets. That also usually sets the max speed of street running light rail trains too. This sort of makes the actually max speed of the streetcars/light rail trains equipment irreverent for street running applications.

LRT is also has 65mph variants which are very common.

True, but I don't recall ever seeing any light rail trains going 65 mph in city streets. You will see those speeds in tunnels and aerial guideways where they are basically grade separated from other traffic. One could possibly build a streetcar going that fast, but why when their speeds are limited to speeds of the city streets? Could a streetcar ever accelerate to those speed before having to slow down for the next stop 1/8, 1/4, to 1/2 mile away?

We're not defining the differences between the vehicles anymore, but rather defining the difference between overall systems. Which is what I've been suggesting all along.

Atlanta's current streetcar project is actually 2.7 miles rather than 1.4 miles. I didn't see where anyone corrected that. :) http://www.atlantadowntown.com/_file...cember2011.pdf

The whole concept of light rail when it was devised back in the 70s was to create hybrid regional systems at a lower cost than full-fledged metro/heavy-rail technology could deliver (BART and MARTA demanded huge sums of money and Baltimore/Miami were cut off before they could build a full system). The result was something that was a mix of streetcar and metro systems. This had precedents in systems like LA's, Philly's, and Newark's, where regular surface streetcars were routed into downtown subways to avoid street congestion.

In the 70s, the situation was reversed - planners realized that many downtowns had extensive downtown street grids that were under capacity, where lanes or entire streets could be given over to surface rail, avoiding the insane cost of downtown subways. In city neighborhoods and suburbs, freight-rail or freeway ROWs could be used to ensure high speeds, which were necessary in order to reach suburban destinations in reasonable amounts of time. These systems would have varying degrees of grade separation depending on the desired train speed and the levels of road traffic at each crossing.

Of course, because light-rail is just a way to describe some form of hybrid, it's impossible to define what IS and is NOT light rail. This is why Houston is building a "light-rail" system that's really just a big streetcar network, and why Honolulu is building a "light-rail" system that's really just a metro with short trains.

The 3 mile Mesa extension of the Phoenix light rail line is under construction.

Also, I'm not sure if you're counting people mover systems, but the Phoenix Sky Harbor people mover is under construction and on schedule to open late next year.

Seattle's First Hill Streetcar line broke ground today.

http://www.seattlestreetcar.org/

FWIW- Construction of Salt Lake's Sugarhouse Streetcar (2.74 miles) is now underway, though the 'official' groundbreaking isn't until next month:

http://www.rideuta.com/mc/?page=Proj...r-Construction

Yes, please followed the link:

This is only for Central Mesa 3.1 mile extension.

http://www.facebook.com/#!/METROrailConstruction

And also, there is more PHX SkyTrain. You can see how is construction in progress.

http://www.facebook.com/#!/PHXSkyHarbor

You can see how is construction in progress. You will see it.

2.7 track miles, which means including the tracks in both directions. Basically double-counting.

The Mesa web site says construction has not started yet. *Design* has started, but not construction. The facebook page N830MH linked to says construction starts on May 30. And no, I'm not going to include airport people movers; some of them might be rail, but we can all agree they're a different animal than urban rail.

Added these three to the list.

Thanks! I saw some construction workers the other day. It's finally started!

Relatively minor but RTD broke ground on the 1.5 mile I225 LRT extension from 9 mile to Iliff today.

They are still in the RFP process to extend it beyond Iliff. That process was initiated after Kiewit's unsolicited bid to finish the rest of that corridor.

I wasn't making a case for LRT to be used at 65mph in city streets. I was simply pointing out that they max out at 65mph, not 55mph. Depends on the model, yes, but the splits between Streetcar max speeds and LRT max speeds should be: 45mph max for Streetcars and 65mph max for LRT. That was all, I was saying.

Salt Lake City Draper extension missing
 

The Salt Lake City UTA 3.8 mile Draper light rail extension is missing as under construction. http://www.rideuta.com/mc/?page=Proj...e-Construction

With good reason. Other then looking impressive on agency maps with superficial connectivity it would really provide very little excepting taking people to nowhere places to other nowhere places. It is MUCH to spend billions of dollars elsewhere on Metra lines. The other three projects (particularly the UP-W and UP-NW improvements) are very worthwhile IMO.

By the way a few years ago there was a press release about extending the Milwaukee North Line (Fox Lake) to have a split branch up north by Rockland Rd. that would extend north to Gurnee. Giving its proximity to Gurnee Mills, Six Flags, Lambs Farm, and Abott Laboratories I thought would have been a real good idea. Haven't heard anything about it since however.

I for one wish Metra would for once focus on increasing train frequency on its more heavily used lines, even if this runs were "limited runs".....that is runs that didn't proceed to line terminus. I think the north line for instance should run on the half hour during the week; and perhaps eeven on Saturday, every hour or so on Sunday would be sufficient. Quit expanding to area where the stations might get 100 riders a day instead increase frequency in the inner metra region to turn metra into a more viable comprehensive transportation alternative.

Ramping up city service with new infill stations and more frequency would probably require electrification. Not that it isn't a conversation worth having since Metra already blows through populated areas that have less than ideal CTA rail access.

Getting UP to seriously discuss electrification is going to be...challenging. Maybe Metra could do a pilot program on the RI first.