Condition monitoring of track geometry from sensors mounted on an in-service vehicle offers continual monitoring of track geometry that can aid track maintenance strategies. Mounting and maintaining a full track geometry recording system on an in-service vehicle is an expensive proposition as the commonly used optical sensors are difficult to keep working in the dirty railway environment. A simpler and more cost-effective alternative is to estimate track geometry using a small number of robust sensors such as accelerometers and rate gyroscopes, from which a worthwhile proportion of geometric quality measures and specific irregularities can be identified. This paper describes the theory and practical results of using a bogie-mounted pitchrate gyro to obtain mean vertical alignment, conditional on the secondary vertical damper geometry. Left and right axlebox-mounted accelerometers can be added to provide short wavelength irregularity, if required. Results from trials on Tyne and Wear Metro vehicles and on a Class 175 mainline vehicle demonstrate effective vertical irregularity monitoring, in particular the ability to monitor vertical irregularity over a wide range of vehicle speeds down to about 1 ms-1, where vertically sensing accelerometers combined with displacement transducers are unable to function correctly.