A method of communicating on-board condition monitoring data from a train to a ground-side database, the method including: repeatedly collecting sensor readings from respective on-board equipment items of the train; forming plural time series for each of the on-board equipment items, each time series being values of a status signal equal to or derived from the sensor readings of the respective on-board equipment item and spaced at a given time interval, the plural time series being assigned to respective groups; on each time interval: comparing the values of the status signals of the plural time series at the current time with the values of the status signals of the plural time series at the preceding time, and producing a data packet for transmission only when at least one of the status signals of the plural time series is a changed status signal in which its value at the current time differs from its value at the preceding time, the data packet including, in a predetermined order, the following contents: the current value of the changed status signal, the current values of the status signals of the other time series of the group which contains the changed status signal, a unique identifier identifying that group, and a timestamp providing the current time; transmitting each produced data packet to the ground-side database; and recording the contents of each transmitted data packet at predefined locations in the database according to their predetermined order; characterised in that, in the forming of the plural time series for each of the on-board equipment items, the plural time series are assigned to the respective groups on the basis that the status signals of the time series of each group are functionally related such that, at at least some time intervals, when the value of the status signal of one time series of a given group changes, the values of the status signals of the other time series of the given group are also expected to change. repeatedly collecting sensor readings from respective on-board equipment items of the train; forming plural time series for each of the on-board equipment items, each time series being values of a status signal equal to or derived from the sensor readings of the respective on-board equipment item and spaced at a given time interval, the plural time series being assigned to respective groups; on each time interval: comparing the values of the status signals of the plural time series at the current time with the values of the status signals of the plural time series at the preceding time, and producing a data packet for transmission only when at least one of the status signals of the plural time series is a changed status signal in which its value at the current time differs from its value at the preceding time, the data packet including, in a predetermined order, the following contents: the current value of the changed status signal, the current values of the status signals of the other time series of the group which contains the changed status signal, a unique identifier identifying that group, and a timestamp providing the current time; comparing the values of the status signals of the plural time series at the current time with the values of the status signals of the plural time series at the preceding time, and producing a data packet for transmission only when at least one of the status signals of the plural time series is a changed status signal in which its value at the current time differs from its value at the preceding time, the data packet including, in a predetermined order, the following contents: the current value of the changed status signal, the current values of the status signals of the other time series of the group which contains the changed status signal, a unique identifier identifying that group, and a timestamp providing the current time; transmitting each produced data packet to the ground-side database; and recording the contents of each transmitted data packet at predefined locations in the database according to their predetermined order; characterised in that, in the forming of the plural time series for each of the on-board equipment items, the plural time series are assigned to the respective groups on the basis that the status signals of the time series of each group are functionally related such that, at at least some time intervals, when the value of the status signal of one time series of a given group changes, the values of the status signals of the other time series of the given group are also expected to change.