Keywords: railway braking, wheel–rail adhesion, low adhesion, braking safety, leading-axle active probing, brake cylinder pressure control.

Wheel–rail adhesion uncertainty is a key challenge for railway braking control and wheel slide protection. This paper proposes a leading-axle active adhesion probing method to estimate the utilized adhesion coefficient at a prescribed slip-ratio threshold during braking. The leading axle is used as a probing wheelset because it first encounters the unknown rail surface ahead. A bounded brake-cylinder pressure excitation is applied to the leading axle, driving the wheel–rail operating point towards the target slip-ratio region while maintaining a safe small-slip condition. Meanwhile, the current utilized adhesion coefficient is estimated online using an unscented Kalman filter based on the coupled longitudinal–rotational dynamics of the wheelset. Instead of identifying a complete adhesion–slip curve, the proposed method records the estimated adhesion coefficient when the leading-axle slip ratio reaches the prescribed threshold. A SIMPACK multibody dynamics model of a four-car urban rail vehicle is established to validate the method under different speed levels and adhesion-transition scenarios. The simulation results indicate that the proposed logic can complete controlled pressure probing, update the utilized adhesion estimate, and limit excessive wheel slip risk.

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