In line with the increasing number of high speed train (HST) commuters and their demand for mobile data, railways operators expore means of providing high speed onboard wireless access. The data from all train cars are sent to a gateway terminal mounted at the roof of the train and an efficient backhaul solution is required to connect the train to the nearest base station. In this paper, we propose the use of unmanned aerial vehicles (UAVs) equipped with free space optical (FSO) communication terminal to provide backhauling for HSTs. We present a composite fading model for UAV-to-HST channel where both atmospheric turbulence and pointing errors due to position/orientation deviation are considered. Based on the derived fading model, we present closed form expressions for the bit error and outage probability. We further investigate the effect of several parameters such as beam divergence angle, displacement deviation variance, coverage distance and UAV operation altitude on system performance.