Constant rotor system monitoring enables timely control and maintenance actions that decrease the likelihood of severe malfunctions and end product quality deficits. Soft sensors represent a promising branch of solutions enhancing rotor system monitoring. A soft sensor can substitute a malfunctioning physical sensor and provide estimates of a quantity that is difficult to measure. This research demonstrates a soft sensor based on bidirectional long short-term memory (LSTM), and a training procedure for rotor system monitoring at high sampling frequency and varied operating conditions. This study adopts a large rotor and bearing vibration dataset. The soft sensor accurately estimates lateral displacement trajectories of the rotor from the bearing reaction forces over a large range of constant rotating speeds and constant support stiffnesses. The mean absolute error (MAE) of the LSTM-based soft sensor is 0.0063 mm over the test trajectories in the complete operating condition space. The soft sensor performance is shown to decrease significantly to a MAE of 0.0442 mm, if the training dataset is limited in the rotating speed range.