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We Asked a Transit Planner How to Up Our 'Mini Metro' Game

'Mini Metro' may look simple, but it has plenty of lessons for real-life transit puzzlers.

Metro fever has gripped Hong Kong. On December 28th, the city opened its long-awaited South Island Line, the first rail system to serve the southern half of Hong Kong Island. Subway enthusiasts thronged the new route on opening day, staring at the public artwork in each station and taking videos out the front of the new driverless cars.

But the new line has its detractors. Though it serves a community badly in need of alternate transport, the South Island Line also promises to feed nearly 170,000 passengers a day into Admiralty Station—a major interchange already known for rush hour congestion. Though all has gone well barring a few power glitches, there are fears that when a fourth line connects to Admiralty in 2021, the concourses will be totally overwhelmed.

Given that Mini Metro is a fairly light, simple game, it's astonishing that it can convey such a nuanced concept about rail planning

This problem will be familiar to anyone who's played the transport simulation game Mini Metro. It's always the way, isn't it? You build a new line to serve a developing area, and suddenly all those new passengers are clogging up your stations and filling your trains. Expanding the network always puts pressure on your established line. Look at the Hong Kong subway map and you can see trouble brewing.

Given that Mini Metro is a fairly light, simple game, it's astonishing that it can convey such a nuanced concept about rail planning. But that's far from the only lesson it holds—in fact, transit specialists are taking notice of the game and how it conveys the challenges, strategies, and frustrations of building a public transportation system.

And it turns out that playing like a transit designer can have very real benefits for upping your Mini Metro game.

"We Didn't Do Any Research What-So-Ever"

But if you speak to Mini Metro's developers, twin brothers Peter and Robert Curry, this resemblance to real transport networks was more serendipity than design. The game started life as Mind the Gap, a browser-based simulator the pair created during a 2013 game jam.

"The inspiration was as much to do with our constraints than anything else," Peter Curry told Waypoint in an email interview. "We wanted to improve the odds that we'd actually finish and ship for once, so we ruled out any ideas we couldn't easily execute."

After listing their assets, they realized that while both of them had programming skills, neither had any artistic ability. Therefore any game they made should have a clean, abstract interface without extensive production art.

Above: the Mini Metro trailer.

"That led Robert to think about planning trips on the London Underground," says Curry. "He'd found it surprisingly satisfying, so perhaps we could make a game with the tube map?"

The game was enough of a hit that the brothers decided to stick with the formula and upgrade the game to a full experience. They formed a studio—Dinosaur Polo Club—and hired a small team to work at their home base in Wellington, New Zealand. After sweeping through IndieCade, Steam Greenlight, and Early Access, Mini Metro released on Steam in November of 2015—and that's when the team began receiving long emails from public transit enthusiasts, designers, and academics.

"At first we were a bit nervous when it went through the transit planning community, because we'd done so little research," admits Curry. "Having experts [pore] over what we'd made was understandably unsettling."

But while there were some corrections and feedback, the overall response was positive. Planners enjoyed seeing a video game portray their work, while the transit map community gloried in a system that let them sketch their own rail networks. One expert even pointed out that the simple simulations of Mini Metro weren't far off from the software high-level experts use to analyze train patterns.

"When you make an oddball game like this one," Curry says, "all sorts of people come out of the woodwork."

A Transit Planner Cracks the Mini Metro Code

One of those people was Jarrett Walker, an author and transit network design consultant who's worked on systems in the US, Canada, Australia, and New Zealand. In December 2014, Walker ran across Mini Metro and wrote a breakdown of the game on his professional blog, Human Transit , claiming it could teach a few sound principles of network design.

"Mini Metro has proven one very important thing," said Walker in an email interview with Waypoint. "Until it came along, the assumption was that transportation games had to have a rich user-interface with pictures of trains and cars and trees and so on. In fact, you can have a game that strips a problem down to its underlying structure, which is what a metro network is."

Header and all Mini Metro screens courtesy of Dino Polo Club

Turning a trained eye to the game, Walker wrote a deep-dive on how the game represents public transit networks. For example, the casual player might look at Mini Metro's network of station and passenger symbols—squares, circles and triangles—as a simple match game. Triangle passengers go to the triangle station, while circles go to the circles. Easy, right?

Walker, on the other hand, interprets these as different districts: circles are residential areas, triangles are retail/activity centers, squares are the downtown business districts full of jobs, and crosses are hospitals. 

"He figured it all out," says Curry, adding that even though this was their original interpretation, the team now only refers to stations by shape. "The only reason they ever correlated to specific destinations was to ease the creation of the passenger timetables. For example, so a hospital-bound passenger will appear at a residential stop at a reasonable time of day."

But far from merely deciphering the symbols, Walker pointed out that these randomly spawning stations follow little rhyme or reason—making the cities of Mini Metro a nightmare for a transit planner.

"Mini Metro imagines a city that is planned by someone else, or that grows haphazardly, where the transit system is totally reactive," Walker says. "A large area that's all residential with few jobs (e.g. circles without squares) is a big problem that will quickly overload your network, no matter what you do."

According to Walker, this poor land use is a worst-case scenario for transit planners, forcing them to either string together residential areas on a single line—and overload it—or build multiple lines into the same area, which gets expensive fast. However, it's also an increasingly familiar problem in cities that have experienced explosive growth in the suburbs.

"The world's great metro systems weren't built that way. They were retrofitted into 19th century cities, but then the city grew around the metro network, so that the fit between development and transit is generally good."

As an example, he cites the Paris Metro—an older system that grew into a grid pattern of intersecting lines as it extended outward. Grid systems, Walker explains, are the most efficient type of rail plan because it maximizes the amount of routes to a destination while minimizing the number of line changes.

Mini Metro

"Visualize a simple x-shaped network," he says, "two lines crossing with one junction station. This means that each line is useful for going anywhere on either of the two lines. So the usefulness of both lines is double what it would be if either of the two lines stood alone. That's the basic multiplier effect of grid connections."

"A grid network is just a structure built out of that pattern, repeated over and over," he says, ideally with lines that run all the way across a city and meet at 90 degree angles. Usually these lines run on north-south and east-west axes, but spider web patterns can work too. "This is why smart cities that aren't [street] grids still try to create grid effects with their network."

As it turns out, this principle holds true for Mini Metro. Barring the maps with major water barriers—where the player has to conserve tunnels—implementing a grid system is an effective strategy for high-level play. (Due to the limited number of lines in Mini Metro, some lines will have to be L-shapes that bracket the city's outskirts.) Like in real transit systems, in-game grid systems give passengers multiple paths for every trip, so station usage tends to even out much like traffic patterns on a gridded street.

As a counter-example to Paris, look at the aforementioned South Island Line in Hong Kong—it's a T junction, meaning the line terminates by ending in the middle of an existing line. That's a sub-optimal pattern, prone to overcrowding.

"The Hong Kong metro is full of T-junctions," says Walker, "where all of the traffic from one line is dumped into another line, partway along the line. This usually means that capacity on the latter line is used very inefficiently."

It's a fascinating piece of modern life mirroring technology and vice versa: you can use a simple, minimalist mobile game to simulate a bleeding-edge transport challenge—while riding the very line you're replicating.

Changing Tracks—Is Mini Metro Really About Rail?

However, Walker also points out that some aspects of Mini Metro aren't as successful at simulating rail networks. Overcapacity works well as a fail state, he says, but in reality when networks get crowded, passengers just choose another means of transport. A much more important metric is whether a metro is able to make enough money to support and expand its operation.

Similarly, the cost units in the game may provide balance, but they're absurd in real-world terms. Forcing a player to choose between a new rail carriage and a new line makes little sense economically, particularly because lines vary in cost depending on their length.

"A carriage is extremely cheap compared to subway infrastructure," says Walker. "Carriages cost around $1 million, compared to subway infrastructure that can run $0.5 billion per kilometer."

Casting tunnels as a rare asset, he adds, is also a bit misleading. These days many "deep bore" subway lines can already pass below shallow rivers, so often water crossings don't cost any more than standard lines. "A more accurate game would have a per-kilometer cost for all line segments, not just water crossings."

But the largest problem with using Mini Metro as a serious city simulator—rather than a fun, thought-provoking game—is the player's ability to edit and delete rail lines on the fly. Rail doesn't work like that. Once cities build a railway, it's more or less permanent.

"Late in the game, you are rewarded for literally building a line that's used once, to unload people from an over-full station," Walker explains. The player is then free to delete the line and rebuild it somewhere else. "Success in the game requires doing that numerous times."

Once cities build a railway, it's more or less permanent.

In response to this criticism, Dinosaur Polo Club added an "Extreme Mode" to the game, where rail networks are permanent once laid down.

"Extreme mode fixes most of the issues that the core game has when you get to high-level play," says Peter Curry, when asked about the change. "But in the process it takes a lot of the fun of creation out. It was our way to make a conceit to balance while maintaining the freedom of the original concept."

However, Walker feels extreme mode creates the opposite problem. It's impossibly expensive, he says, to build a network in response to random patterns of demand growth while remaining locked into past choices. "That's not realistic at all."

Yet, Walker argues that all these problems disappear if the player makes a mental track change, and instead imagines that Mini Metro represents a bus system.

Unlike rail networks, bus systems can adjust their routes on the fly in order to meet demand, add stops, and even schedule temporary routes to avoid overcrowding. In an ideal real-world transit system, cities would start out with bus systems and—after years of analyzing traffic patterns—eventually lay rail lines on the most popular routes. This essentially happens in the game, Walker points out.

all these problems disappear if the player makes a mental track change, and instead imagines that  Mini Metro represents a  bus system.

"Over time, successful portions of your Mini Metro network do become permanent, in the same way that bus service patterns become permanent when they arrive at durable structures like frequent grids."

Critiques aside, Walker considers Mini Metro to be a major step forward in transit simulators because it's approachable and easy to draw lessons from. He hopes that in the future, new games will build on the game's foundation, while providing a more realistic approach to cost elements.

"I would be keen to consult with anyone conceiving the next generation of these games," Walker adds. "There is a need for games that are as fun as Mini Metro, but more accurate, so that people learn interesting things as they play."

Will There Be a Mini Metro 2?

After three years of continuous work on Mini Metro, Peter Curry betrays a hint of exhaustion when asked whether there are plans for sequel.

An endless mode simulation game, he explains, is tricky to balance. Given the lessons they learned in development, he's unsure whether the studio would tackle one in quite the same way again. But he points out that Dinosaur Polo Club has continued to add content to the game—including new maps like Shanghai and Seoul—and hunt for bugs in the passenger AI. Curry also notes that many players still haven't discovered the game's less-explicitly explained features, like the fact that you can stop time by clicking on the clock, then edit lines while paused. (Another Easter Egg for transit nerds: each city map is ordered by the opening date of its metro.)

But despite this deluge of updates, they have considered a sequel.

"Cripes," says Curry. "I can't say that we haven't talked about it. We're not in a position to say what we'll be working on next just yet—there's a lot of work still left on Mini Metro."

That's not a yes, but it's not a no either. And in the meantime, Peter Curry can never see subway maps the same way again.

"When planning a route, I do find myself thinking on how we could use that particular map's graphic design elements in the game, and make it interactive."

So hold onto that extra line—you may need it.

Robert Rath is a freelance writer, novelist, and researcher based in Hong Kong. His articles have appeared in Zam, Vice, The Escapist, Playboy and Slate. You can follow his exploits at RobWritesPulp.com or on Twitter at @RobWritesPulp.