About four decades ago, Baltimore built the first line of what was supposed to be an entire subway network before a lapse in funding shelved plans for expansion.

Now it’s your turn to finish it.

“Subway Builder,” a new simulation game, allows amateur transit planners to remake the rail system in Baltimore and 25 other U.S. cities. Players start with a budget — or can play in a budget-free sandbox mode — to build Metro lines and attract commuters across the city. It’s all modeled on U.S. Census data.

And trust us, transit nerds, this will be your new favorite game. (A public beta version is being released Thursday.)

(Follow The Banner on Twitch — we’ll stream the game next week!)

“I hope people realize that, right now, the state of transit expansion in the U.S. is pretty dire, and I hope that people realize that better things are possible. We can build new stuff,” said Colin Miller, creator of “Subway Builder.”

In the game, players see their city of choice from an adjustable bird’s-eye view (think Google Maps), complete with the road network, buildings and some topography. Players start by placing stations, which can be on the surface or at specific depths underground, impacting cost; connecting them with track; and selecting how frequently trains stop based on time of day and the number of train cars available (another variable impacting the bottom line).

Then hit play, and little red dots start inching toward the stations from different population and employment centers, and gray blocks — representing heavy and light Metro trains — shuttle them along. The number of riders even goes up as rush hours hit.

The goal: Make the system turn a profit — a tall order for any real public transit agency. Most across the U.S. don’t, recapturing just a percentage of operating budgets through ticket sales. The Maryland Transit Administration ranks toward the low end, making back roughly 10% of its operating budget at the fare box, according to the latest federal data.

Baltimore once had an expansive trolley system, with streetcars traveling from downtown to Ellicott City, Towson, Woodlawn, Carney and Sparrows Point. That system passed with the rise of personal vehicles and buses, which weren’t limited to the rail lines.

The state tried to develop a Baltimore subway system like Washington, D.C.’s, but soaring costs and limited funding resulted in just one line opening in 1983 from Owings Mills to the Johns Hopkins Hospital.

To keep things going, officials switched to a less expensive light rail proposal using an abandoned rail right of way to build a line in the 1990s that eventually stretched between Hunt Valley and Baltimore-Washington International Thurgood Marshall Airport.

In “Subway Builder”s regular game mode, players get $3 billion to start and can add on to their system as they make money. The game uses “Mediterranean pricing,” meaning it calculates costs based on the real-world estimates of costs per mile overseas.

But why not U.S. prices?

“It’s the only way the game would be playable,” Miller said.

Building transit systems in the United States costs a lot more than in other parts of the world. Like, a lot more — double, on average, and as much as five times more in New York City.

The sandbox mode (no budget constraints) is a slippery rabbit hole totally worth falling down. Thanks to an advanced copy of the game, The Baltimore Banner built an updated version of a rail plan proposed for Baltimore in 2002.

Yes, Baltimore transit nerds, we built the map. We even added on to it to account for two decades of development. It only cost — checks screen, swallows — $20 billion. In the U.S., call it $40 billion. Then add right-of-way considerations, the red tape and the politics of it all.

OK, not fun anymore.

Miller said the idea for the game came to him when he was stuck on a delayed car on New York City’s subway. After searching for a game like “Subway Builder” and coming up empty, he decided to build it himself. He figured it would take him a few days.

When the clip he posted of his demo went viral, he decided to spend months building out a full game.

The 21-year-old software engineer borrowed from his coding background and the data sets he uses in his day job as the founder of a startup that leverages data to help political campaigns build maps and craft strategies. In the game, demand for transit is modeled on details of real-life commutes to and from work collected by the U.S. Census Bureau. It even includes a route-planning tool that shows users how long it would take to get from Point A to Point B on their dream subway system.

The launch version of the game features 26 cities. In a midsize metropolis like Charm City, players will have to pay extra attention to their budget and the demand for transit.

New York City is “easy mode,” Miller said — it’s dense enough that you can build a line just about anywhere and riders in the game will flock to it. In smaller cities with less population density, it’s tougher to turn a profit.

Miller plans to roll out additional updates with more cities and new game mechanics, including a growth feature, which will increase population density over time near station areas to simulate how transit spurs development, which in turn adds new riders.

Download this map of the current light rail and Metro lines to get started.