Just a few years ago, electric buses routinely faltered in cold conditions, reinforcing doubts about whether they could replace diesel and natural gas-burning fleets in northern cities. Now, with better batteries and strategically placed chargers, Madison is at the forefront of a small but growing number of cities testing whether those doubts still hold. Making the technology work through a long Midwestern winter could reshape how others approach electrification. Some 3.6 million commuters nationwide rely on buses to get around. With transportation accounting for roughly 28 percent of the nation’s greenhouse gas emissions, transit agencies are looking for alternatives to polluting machinery that creates a particular health risk around bus stops.