LEGO Trains

Lego Trains is a Lego theme seen for the first time in 1966. It doesn't count as a Long Runner as much as LEGO City, however, as it has been completely redesigned several times.
 * The first trains from 1966 had either no motor at all or a 4.5V motor fed by three C-cell batteries on the locomotive itself. The rails were blue, and the sleepers were white.
 * 1969 brought magnetic couplers and the 12V system with a pair of electric rails in the middle of the track. Lego trains could finally be remote-controlled like real model trains.
 * 1980 saw a major theme reboot and the transition from the Blue Era to the Gray Era. There were still unmotorized, 4.5V (with the batteries in a separate car since 1972) and 12V trains, but first of all, the rails were gray, and the sleepers were dark gray now. Trains could be illuminated with 2×2 lightbricks now (which were pre-installed only in the biggest train sets, 7740 and 7745). Multiple trains on the same layout were possible now because they could be stopped at remote-controlled red signals. Other than these and points which had already been remote-controlled in the Blue Era, later additions with remote control were a decoupler and a level crossing. Last but not least, all trains were now compatible to the minifigs introduced in 1978.
 * Most of this was discarded in 1991 when the new 9V railroad was launched. Its major advantages were the absence of the third and fourth rails in the middle because the electricity was now picked up from the outer rails, and the wheel bearing/suspension imitations outside the wheels. The downside was that almost all model railroad features had been canceled. There were no remote-controlled points anymore, nor were there remote-controlled signals at which to stop a train so that only one locomotive could be on a layout at a time. As if that weren't bad enough, LEGO quit making replacement parts for the 12V line, which soon started to run out of lightbricks, traction tires and motors.
 * Within one year, starting in 2006, the 9V system was replaced with something even cheaper and simpler: battery-powered, radio-controlled trains. The battery pack made MOC trains more difficult, but on the other hand, terminal loops were possible for the first time since 4.5V was discontinued, and actually running multiple trains on the same layout independently was possible for the first time (at least without converting 12V trains to digital controls).
 * Yet another battery-based system was introduced in 2009, probably also to appease the AWOLs, mostly on whose money Lego Trains lived after all. It is called Power Functions Trains, infrared-controlled, it works on technology already established in Lego Technic, and for the first time, decent large steam locomotives. Besides, Power Functions allows for running four trains independently at once and two functions to be controlled on each train (alternatively, up to eight trains with only the motor controlled). Power Functions Trains is not only the true successor to the 9V system, but it's planned to replace RC Trains as well.

Something worth mentioning is the up-and-down in model quality. The 1980 12V system introduced life-like (for 1980s Lego standards, that is) trains, often based on real-life rolling stock. During The Nineties, however, more and more 9V sets looked nothing like any train at all. Then came the Super Chief, one of the first Lego Trains sets marketed at AFOLs directly. Soon, the futuristic fantasy designs were discarded, and trains looked like trains again.

Some Lego Trains tropes

 * All There in the Manual: There was a trick to keep the short-lived 12V lightbricks from dying in masses when used for stationary illumination, and that was to connect two of them in series and thus run them on 6V only. However, the only source for this information was the Lego staff.
 * There's no information whatsoever about what to do when you run out of 12V traction tires (other than to convert your whole layout to 9V). The solution, namely to cut pieces out of inner tubes for racing bicycles (which is also cheaper than Lego's original traction tires ever were), is available from users only.
 * Art Shift: Lego trains changed their looks many times, but in 2002, Lego caught up with AFOLs at modeling and presented the Super Chief which hardly looks like a toy anymore with its SNOT front end and handrails. And the Fandom Rejoiced. They did it again in 2009: The Emerald Night conceals almost all of its studs.
 * Audience Shift: While there are still entry-level sets, Lego has been targeting their cool, big train sets at AFOLs directly for almost ten years now.
 * Brand X: The nameless railroad company introduced with the 9V trains in 1991. Also, Octan tank cars.
 * On the other hand, Gray Era 12V trains could be fitted with stickers from all European national railroads, and 9V trains proudly wear American railroad names and liveries.
 * Cool Train
 * The 7740 Trans-Europ-Express, the biggest set until the launch of the 7745 high speed train which it outshines nevertheless. Also because the locomotive was the only Lego vehicle with pre-installed illumination at that time.
 * The Santa Fe Super Chief, sets 10020, 10022, and 10025. A train that looked like modeled by AFOLs. Since the locomotive was sold separately, many fans bought several ones, sometimes even enough to build at least one B unit, and run their Super Chief multi-headed. Other fans managed to build Fan Remakes in exactly the same shape but different liveries such as a California Zephyr with locomotives in the Denver & Rio Grande Western livery.
 * The Hogwarts Express. Even the one with 12V wheels and without motor, rails, and tender. Even more so when modded with parts from an Emerald Night locomotive.
 * The 10194 Emerald Night. The first train that's ready for Power Functions. The locomotive is Lego's very first Pacific and easily the best-looking Lego steamer ever made. Also, this set mostly avoids to leave any studs visible. Since it comes with only one car, many AWOLs buy two or more and use the spare locomotive parts to build their own steamers or convert their 9V Hogwarts Express locomotive to something that resembles a Castle class more closely—in fact, since the Emerald Night is sold without tracks and usually unpowered, it's quite reasonably priced.
 * Since we're talking about Lego, there are countless MOCs to which this applies, too.
 * Crack is Cheaper: Especially discontinued, sought-after sets. Don't even consider buying a 7740 with working motor, working (let alone original) lightbricks, traction tires, and all stickers in place. Compared to this, the (motorless, trackless, one-coach) Emerald Night is cheap.
 * And still, AFOLs keep buying the newest, hottest, and biggest train kits several times.
 * Cult Classic: The 7740 Trans-Europ-Express and the Super Chief.
 * Dork Age: Starting in 1996 with the introduction of the 4559 Cargo Railway, most Lego trains looked like anything, but not like trains. The My Own Train series and the re-release of the Metroliner of 2001 and finally 2002's Super Chief ended this period.
 * Guilty Pleasure: It's already difficult to explain having a model railroad. Now try and justify your huge Lego layout. Makes one wonder what people say who go into stores to buy three or four Emerald Nights at once ("Eh, I've got four nephews...").
 * Genre Shift: Several times. In 1991, LEGO Trains quit being a model railroad technically (which it had been since 1980 when points and signals could be remote-controlled) when switching to simpler tech. Eleven years later, LEGO Trains suddenly turned into a model railroad style-wise and copied some superior modeling methods developed by AFOLs.
 * Germans Love David Hasselhoff: Apparently, the 7740 train set—based on the German class 103 with German Trans-Europ-Express coaches—is quite popular in the USA.
 * Just Train Wrong: Can be applied when Lego tried to model a real-life train.
 * The Gray Era trains all came with stickers of all major European railroads, no matter what they were based on.
 * Averted by most 7740s based on a German TEE train. There were only DB stickers for the cars anyway, so most locomotives were correctly given DB stickers, too.
 * It seems like remarkably few 7745s (based on the French TGV Sud-Est) were labeled SNCF trains, though.
 * Of all train destination stickers for 7740, only Basel–Hamburg and maybe Wien–Zürich (in case the train would run via Munich) would have been realistic.
 * There have never been any mail (7820, 7819) or sleeping cars (7815) in Trans-Europ-Express trains consists, let alone in this livery.
 * A restaurant in one of the end cars of a TGV (7745)? What the? On the other hand, having two power cars like in Real Life would have made the train too long and too expensive.
 * The Metroliner combines an Amtrak livery with side buffers. The BNSF freight locomotive has them, the TTX container car has them, and the Super Chief lacks buffers on the locomotive's cab end only. In Real Life, American railroads have no side buffers, however.
 * Then again, Lego also put side buffers between the locomotive and the tender in the 7750 set.
 * The cheapest Hogwarts Express comes without a tender.
 * The Merch: Lego's signing countless merch contracts doesn't leave out the trains. First, Lego's Harry Potter product line included several versions of the Hogwarts Express, and then came the Toy Story line of merch which included a train, too.
 * Periphery Demographic: Subverted. Lego knows pretty well by now that it's mostly AFOLs who buy their best train sets. See Audience Shift.
 * Show Accuracy: Lego could have done a much better job at modeling the Hogwarts Express, had they revived the large train wheels from 7750 and made them black (since the trackless versions already run on 12V wheels), not used the 6×28 train base plate for the locomotive, and given all versions tenders and at least two cars, thus making it an actually reasonable toy train instead of a toy movie prop. Yes, it would have been more expensive, but we're talking about Harry Potter merch, so there's no such thing as "too expensive." It'd be interesting to see how a Lego Hogwarts Express would look like as merch for Harry Potter and the Deathly Hallows, designed by the same people who brought us the Emerald Night, other than still having only one car.
 * The 7597 Toy Story train set turned out much more decent and credible, also because it's a toy train that's supposed to look like a toy train.
 * They Changed It, Now It Sucks: The switch from 12V to 9V was viewed this way back then.
 * And again with the switch from 9V to R/C. And then reversed with the well received change from R/C to Power Functions.