New York City’s mayor-elect Zohran Mamdani’s promise to make the biggest public-bus system in the US free has investors of the Metropolitan Transportation Authority’s $17 billion of farebox bonds watching closely since that bus revenue is used in part to repay the debt.

Mamdani, a democratic socialist who won Tuesday’s election, has proposed raising taxes on millionaires and businesses to bring in $9 billion of new revenue to pay for the free buses, along with free childcare and building more affordable housing. He’s said New Yorkers need free transit options to help make the most populous and costliest US city more affordable.