As 2,000 new residential units come onto the market on Roosevelt Island over the next few years, the island’s growing population faces some public transit challenges. Next year, the tram that connects the island to Manhattan is set to close for at least six months for a $25 million dollar repair and overhaul. The timing isn’t good, as several hundred new residents are set to move in to new developments. Hudson Properties and Related have completed four of the seven high-rise condos they plan to build there; they are mostly home to hospital employees and New York University faculty. Another development, Riverwalk Court, will finish by the end of the year.