The ride-hailing app company raised another $1.6 billion Wednesday with a sale of convertible bonds to clients of Goldman Sachs & Co. (GS), according to various media reports. Preparations for the offering had first been reported by Fortune in December.

That adds to around $2.8 billion raised so far in equity, according to the Wall Street Journal, the last $1.2 billion of which came only two months ago, when internet giant Baidu Inc. (BIDU) injected an estimated $600 million to help roll out Uber’s services in China.

The latest round of equity funding valued Uber at $41.4 billion, briefly making it the most valuable start-up ever (it was eclipsed shortly afterwards by Chinese smartphone maker Xiaomi).

The WSJ said Uber had offered investors a six-year bond that converts into equity at a discount of 20%-30% to the price of Uber’s initial public offering, whenever that takes place. The Financial Times reported that the coupon on the bond will rise if the IPO doesn’t happen within four years. The timing of that step-up is interesting because it gives the company freedom to stay private without any increase in its cost of capital for four years, whereas many observers expect it to IPO well before that.

The company faces considerable challenges in rolling out its taxi-hailing services, amid concerns over passenger safety and resistance from entrenched vested interests among licensed taxi drivers. Resistance has been particularly fierce to it in Europe, notably in Germany, where licensed taxi drivers have successfully challenged Uber drivers’ right to pick up passengers.

CEO Travis Kalanick said in a speech in Munich last week that “We want to make 2015 the year where we establish partnerships with new European cities.”

While it’s the taxi-hailing app that has been the cornerstone of Uber’s growth to date, the company is now branching out into car-pooling, having already debuted the service in New York, San Francisco and Paris.

Learn more of the latest Uber news from Fortune’s video team: