We estimate the network externality of a public transit system by examining the effects of its expansion on the housing market. Our results show that a major expansion of Singapore's Mass Rapid Transit (MRT) system increased the price of apartments within 0.5 km of a pre-expansion station by 1.6% to 2.1% relative to apartments that were further away from a station. Evaluated at the mean housing price, the expansion increased the value of pre-connected apartments by at least S$386 million, which is equivalent to about 8% of the estimated S$5 billion cost of the expansion.