The initial network was built by tying underused lines into an existing Underground[1] line, the East London Line. Parts had to be upgraded, but the expensive bits were already done - there were existing stations, and existing, almost completely grade-separated, electrified double track. There were upgrades to platforms here and there, and some connecting track built, but basically what the Overground did largely was improve service patterns, frequency, wayfinding, and ticketing.
I don't know if many other cities have this kind of infrastructure sitting around and not being used to its full potential. Philadelphia's SEPTA Regional Rail is probably one. Toronto's GO has the trackage and the stations, but hundreds of route km need to be electrified.
[1] The ELL was a "subsurface" Underground line, like the District Line, Metropolitan Line, and others. Those lines use basically full size commuter trains, and have air conditioning. This is in contrast to deep-level tube lines like the Central Line and Bakerloo Line that have narrow trains with a round cross-section, in narrow tunnels.
Overground is a great example of how to re-use existing rights of way, but I do think London is a bit in the sheer number of under or dis-used lines that were available. DLR is a similar re-use of existing rights of way, but there just are not that many places that had such a huge explosion of rail in the 19th century that is now derelict and available for re-use.
I do think Overground is a great case study in the value of both branding and service. Quite a bit of the Overground was already operational passenger rail, but it was fragmented under different operators, names, and service patterns. TfL wisely brought it under a single brand, making it simple to understand, especially as part of the larger London transportation network.
TfL also arranged consistent service, newer rolling stock, and full electrification and made the service pretty reliable. Turns out if you offer good frequency people will make use of it. Wild.
Here in Ontario we've been watching Metrolinx try and fail miserably to build out an RER/S-Bahn system out of the GO network. We're 8 years into the GO Expansion plan and Metrolinx has yet to raise a single catenary pole. It's beyond shameful. We're getting tons of shiny stations with huge capacity, but no real service has emerged (or shows any sign of emerging) to get value from all the dollars spent.
Metrolinx has been unwilling to make the leap to metro operations with line isolation and high floor platforms. They are also unwilling to move to electric multiple units to realize acceleration gains. They are trying to cling to scheduled service and commuter-oriented patterns, just because that's how it's always been done.
The ELL was a tiny part compared to the NLL, Watford DC and Goblin and about the same size as the WLL.
The thing which TfL broggght was sprucing up the trains, adding staff to the stations, increasing reliability if I think frequency, and branding it so people considered it “new”.
Branding also underscores a thing that transit nerds would know but is not necessarily obvious to visitors or infrequent users - that these are just part of the same transit network and so there's no extra friction. You don't need a special ticket or whatever - an Overground train, just like London's Underground trains or buses or trams, obviously works with Oyster, it has the common fare system, if your journey involves a mix of modes that'll integrate smoothly and so on.
I don’t think oyster worked on overground when TfL took over, other than the special sections where there was co-acceptance (harrow-queens park, Richmond branch). Of course in those days most regular users had a travelcard.
Oyster, and later contactless, makes things far easier to travel. I’ve just been on a u-bahn and bus trip in Nuremberg, had to download an app to buy a ticket, no idea whether I got the right one or not.
Oh, I see, you mean maybe when it's "Silverlink" your Oyster might not work at a barrier and then maybe a month later it's "Overground" and now it works?
Maybe. There are definitely cases where the re-brand is accompanied with physical works that make access legal, but I couldn't find cases where those works change practical access. If I have an Oyster card, and I walk up to a station with no barriers and board a Silverlink train that "doesn't take" Oyster, it does work it just wasn't legal. A month later with new "Overground" branding, that's legal and they've installed a barrier-less validation terminal so I could and should tap it with the Oyster as I pass.
I looked for, but couldn't find, pre-Overground stations where there's a NR style gate line which is physically capable of reading Oyster, but Oyster is forbidden and so you can't enter until - with the re-branding - TfL turns on the Oyster mode. I think it's likely that did not happen because it just seems pointlessly annoying, but I can't prove it.
Oyster came in as travelcard only, and was valid on all in zone transport including the pre-tfl North London Line etc.
It wasn’t valid to say Watford though.
When PAYG was introduced it was not valid on trains (other than a couple of exceptions)
But You’re right, Oyster PAYG came in the day TfL took over from silverlink - 11 November 2007. It was 2010 before it came out across the whole travelcard area on rail services. That was because the other TOCs didn’t like the idea and felt they’d lose out on revenue.
> Oyster came in as travelcard only
I don't think this can be correct. One of the early Oyster "problems" was that people who'd routinely made some particular journey on their travelcard, perhaps for years, would get locked out of Oyster and be unable to start a second journey. The reason was that their Travelcard had never been valid for their journey but it was valid for both the entry station and exit station and before Oyster it had no way to connect the dots, now Oyster could see the journey route wasn't allowed and so it charged them, making their balance negative, whereupon they can't start a second journey until they pay for the first.
Edited to add: Oh wait, maybe you meant only on (what would be) Overground stations ? That seems very weird, but I guess I can't prove you're wrong.
First was staff travel, then annual and monthly travel adds, then Prepay (later PAYG) on the tube, then bus, then overground, then later suburban trains.
If you had a z1-3 travelcard you could travel to zone 6 with Prepay as long as you had a positive credit and would have the allroaite fare deducted. For travel on train you had to buy an “extension ticket” though. That I think might have extended even past payg availability on trains for a while.
It’s amazing in hindsight how quickly things changed, but I remember the various messes were quite notable for the first 10 years of oysters existence.