The first we saw of these new trains was in September 2008 when a mock-up tube carriage was displayed outside Euston Square station for the public to inspect. At this stage it was expected that the first new train would enter service in early 2010, a date that later slipped to the end of July.
Air-con was the big selling point for passengers, who on certain days each summer endured sweaty rides that would now cease. But the big selling point for TfL was capacity, because longer trains with fewer seats meant more people could climb aboard, helping rush hour commuters to get around more easily.
Replacing all the 1960s trains on the Metropolitan line took two years, with the last 'A Stock' running in public service in September 2012. Attention then switched the other sub-surface lines, where the rush hour crush was much worse, with the Hammersmith & City line first to see longer trains.
Six-car 'C Stock' was the target, with complete replacement on all three lines achieved in February 2014. Few mourned the disappearance of these clunking workhorses, and it's now much easier (and cooler) to take a ride.
Back in 2006 it seems the intention had been that only District line trains running to Edgware Road would be replaced. But that aspiration was swiftly extended to cover the whole line, even though the fleet of 'D Stock' units had only recently been refitted.
Since January 2015 these District line stalwarts have been removed from the network at a rate of roughly one a week, trailered off for reuse or scrap, with a scheduled removal deadline in late 2016. New 'S Stock' trains have dripfed into service to replace them, and a couple of weeks ago the last of the new sub-surface fleet of 192 trains was delivered.
So, ten years on from the initial announcement of total rolling stock replacement, are we there yet?
Not quite, and the issue is signalling.
TfL's attempt to upgrade the signalling on the sub-surface lines hasn't been going well, in fact it's been a disaster. The first contract faltered, then collapsed, with expensive repercussions. The latest contract is going much better, but will cost much more and is running several years behind the original schedule.
As a consequence, all 192 S Stock trains are having to return to the factory to have a new in-cab signalling system installed. They're being taken off to Derby a few at a time, and coming back future-proofed, but while they're away TfL needs spare trains to make sure it's always possible to run a full service.
And that's why, even though all the old 'D Stock' units were supposed to have been removed by now, a few remain. I understand ten old trains are theoretically available, but no more than five are generally out in service, and often it's rather fewer than that.
Almost all the trains you'll see out on the District line are new ones, halogen lamps blazing, with higher capacity and fewer seats. But stand for long enough on a District line platform and an old train will eventually turn up. It just might take a while.
I tried catching one the other day, and failed. An old 'D Stock' pulled into Bow Road on the opposite platform, which I couldn't reach in time, and then I saw nothing but 'S Stock' for the ensuing hour. The following day a 'D Stock' pulled in at West Ham running west, but I was going east and had no time for a detour.
Expectations are that these few old trains will continue in service until February next year, when presumably they'll be withdrawn in a blaze of congratulatory publicity. In the meantime, if you'd like to take a final nostalgic ride on the District line you have a couple of months, and you'll need some luck.
What's impressive here is that TfL announced a decade-long rolling stock replacement project ten years ago, and ten years later it's virtually complete. They're a heck of a lot further behind with the signalling, and the regenerative braking, and the improved service pattern, but it's good to know not every mammoth public transport project faces inevitable delays.