The presence of planes, although incipient, on the rolling surface is the cause of a rapid deterioration of the wheel-rail contact with the consequences that this entails both for the environment and for the rolling stock itself. That is why the detection of these planes, before they become apparent due to their consequences, is of great interest and reason for study. This paper presents some of the results obtained in an extensive measurement campaign carried out on a line of the Barcelona Metropolitan Railway. The vibration of the rail at the passage of the wheels is detected by an accelerometer located at the foot of the rail and the exact moment when the wheels pass through the section where the accelerometer is located is determined by the shear deformation of the rail web. Different indicators are proposed and applied to the experimental records to quantify the severity of the planes. One of the aspects analyzed from the experimental data is the variation in the energy peak of the vibration detected as a function of the distance between the section where a plane hits the road and the accelerometer. This peak must be normalized to correctly establish the severity of the plane.