We went from zero to a fairly good system of light rail and subways in twenty years:
And we are still expanding it at a rapid rate. I am a transit buff, but I can’t think of anything comparable.
The DC Metro had four of its five current lines constructed in the span of 14 years (1969-1983; the Green Line didn’t open until 2001.) The Mexico City Metro had 8 lines, 125 stations, and 87 miles of track constructed in a span of 20 years (1967-1987).
Well, the Transcontinental Railroad was built in 6 years.
Why would you even think “in history” with that question? Look East.
The bulk of Hong Kong’s MTR was built in seven years (75-82).
The backbone of Singapore’s MRT was built in five years (83-88).
Kuala Lumpur’s overhead light rail system was built in five years (90-95).
However, the most impressive infrastructure achievement of all (IMO) is Chek Lap Kok airport in Hong Kong, which comprised:[ul][li]Creation of a 12.48 square kilometer artificial island[]Construction of the largest enclosed space on the planet[]Building of one of the longest suspension bridges in the world (road/rail)[]Building a second road/rail suspension bridge[]Creation of two entirely new city suburbs[]Building of a high-speed rail link from the airport to the city[]Extension of the MTR system (in parallel to rail link)[*]Creation of freeway from the city to the airport[/ul]All of which took seven years (91-98).[/li]
All of the above are from first breaking ground to official project opening. You may have different criteria. Whether you agree or not, these achievements are certainly more impressive than Boston. 
The Shanghai Metro has got to be close to a record-holder there. The first line opened there only in 1995, and it’s already the world’s largest subway system.
Why the crack about Boston? It had the first subway in the US, and its construction has always been hampered by having to fit into an existing old city with a highly problematic geology.
I don’t agree that the LA Metro system is “fairly good”. LA is too spread out - millions of people who were taxed to build it live miles and miles from the nearest line. I used to take the Green Line from Norwalk to El Segundo, but I had to drive from my house about 10 miles to Norwalk, and then wait 5-10 minutes for the train. Then I had a 10 minute walk to my office. It ended up taking about five minutes longer on the average than if I drove on the 105 or 91, which if you are familiar with the 105 or 91, is really shitty. I only did it because sitting in traffic really stresses me out, and gave up on it after a year or so. I still have a pile of unused MTA tokens.
Has it made an appreciable dent in traffic?
To be fair, that’s not the best criterion for judging the success of a transit system. After all, the Tokyo transit system is enormous, very comprehensive, and carries huge numbers of people, yet the traffic on Tokyo highways is still jammed every day.
Rather, the question is whether it provides a useful alternative for a large number of people.
It might work better if they’d let slime mold design the system instead. 
Well it was a crack about the Big Dig but I realise that was the opposite of mass transit, so I’ll withdraw it.
Subway construction doesn’t seem any better. They’ve been working on adding new entrances and a pedestrian tunnel across the street to State Street Station for over five years now. I think the actual subway work is done and they’re finishing up putting the streets and sidewalks back together now, but damn, it’s taken forever to get to this point.
Don’t feel bad. It’s taken us nearly a century just to get started on the Second Avenue line.
With all due respect, the Green Line is not the best measure of the usefulness of LA Metro Rail as a whole. It’s the only line that doesn’t go downtown, and it was primarily intended to serve the aerospace and defense works around El Segundo before those industries’ post-Cold War decline. It also goes near, but not to, LAX. :smack: In short, it’s the orphan line of the system, somewhat of a “line to nowhere” compared to the other routes.
The weekday ridership numbers from Metro’s website are:
Blue: 90,109
Green: 45,259
Red/Purple: 171,163
Orange: 42,900
That sounds like a lot of people find Metro Rail, and particularly the Red/Purple line which is a true metro/heavy rail/subway, to be “fairly good”.
For comparison* or contrast, the weekday ridership numbers for CTA rapid transit in Chicago are:
Blue: 165,974
Brown: 101,171
Green: 67,317
Orange: 55,172
Pink: 30,539
Purple: 41,185
Red: 248,826
Yellow: 5,664
I wouldn’t judge the usefulness of the L by the Yellow Line, which like LA’s Green Line doesn’t go downtown.
*Not direct comparison between LA and Chicago, as I don’t know whether LA Metro is using total unlinked trips (which double-counts a person who commutes both ways by the same route, as most people do) like CTA does.
Those passenger numbers are revealing.
In judging how much of dent they do make to traffic you may consider that the Paris metro and London underground each carry 4-5 million passengers per day (for similar metropolitan area populations). So there seems to be a massive potential for both Chicago and LA. As always though it is a cultural change and those tend to be tricky.
Part of the problem – that is, why isn’t L ridership higher – in Chicago is that some of the densest-populated neighborhoods along the lakefront are beyond an easy walk to the L once you get south of the Lakeview neighborhood; say, Lawrence or Wilson Avenues. (North of there, the lakefront is also densely-populated but the L is closer to the lake.) These neighborhoods are served by several express-bus routes that go downtown via Lake Shore Drive.
On an average weekday, CTA bus boardings are 963,511 while CTA rail boardings are 715,847; that is, the bus system carries significantly more people than the rail system. By contrast, Washington (D.C.) Metro’s average weekday boardings were 743,961 for rail and 417,270 for bus. A rougher but more telling contrast is New York City Transit Authority, which has carried in 2011-to-date over a billion rail passengers and under 400 million bus passengers!
A complicating factor – one beyond my ability to find an easy statistical answer while I’m supposed to be working 
– is that Metra commuter rail plays a significant role for in-city trips in Chicago; that is, Metra fills some of the same niche as the L. As I recall, the busiest non-downtown Metra station is Ravenswood, a city stop, and the Rock Island and Electric routes have several stations a half-mile or less apart in southern parts of the city, resembling more a rapid transit line than a commuter line. For example, Hyde Park is a densely-populated South Side neighborhood with several Metra Electric stations right along the lakefront but barely skirted by the Green Line.
You forgot the Gold line.
One big difference is that in London and Paris the rail transit systems are comprehensive – they go to pretty much every neighborhood in the cities. In Chicago and especially L.A., they are far from comprehensive.
But L.A. is building more and more all the time. And for the places they do go, the trains are well used.
To answer the OP’s question, however, it is by no means the fastest in history. Many other transit systems were built much more quickly.
Oops, the number I listed for the Orange line is actually the Gold. :smack: LA Metro lists the Orange line with the bus statistics because the Orange line is, umm, a bus.
Very true. There may well come a tipping point when it becomes the transport method of choice but there is always that chicken and egg situation regarding infrastructure and passengers.