This application describes methods and apparatus for monitoring of rail networks using fibre optic distributed acoustic sensing (DAS), especially for condition monitoring. One method involves taking (902) a first data set corresponding to measurement signals from a plurality of channels of at least one fibre optic distributed acoustic sensor (100) having a sensing fibre (101) deployed to monitor at least part of the path of the rail network (201). The first data set corresponds to measurement signals acquired as a train (202) passed along a first monitored section of the rail network. The method involves identifying (903) a speed of the train through the first monitored section and dividing (904) the first data set into a plurality of time windows. Each time window contains a different subset of the first data set, with the measurement signal for each successive channel in a time window being delayed with respect to the previous channel by a time related to the identified train speed. For each time window, any appropriate time shift is identified (905) and applied (906) to the measurement signals for a channel so as to substantially align the measurement signals of the channels within the time window. The data from the time windows is then combined (907) after any time shifts have been applied to form an aligned first data set; and a characteristic train signal is derived (908) from the aligned first data set. The characteristic signal may be removed from the aligned first data set (1007) to leave remainder data. The characteristic trains signal and/or remainder data may be analysed for condition monitoring.