The underlying principles are considered plus attractions and limitati
ons on the practical application of three different types of longitudi
nal rail profile measuring equipment which can be used in a vehicle. A
xlebox accelerometer systems are a good compromise for obtaining stati
stical profile data at line speeds. Other accelerometer-based systems
are the best means of obtaining data at a few metres per second but ar
e corrupted by extraneous vibration. Both these and chord-based system
s are compromises in most practical circumstances where there is signi
ficant extraneous vibration.