Under what condition is a train least likely to skid when stopping from 30 MPH?

A train is least likely to skid when stopping from 30 MPH if the track is well polished and dry. This combination provides the best possible friction between the train's wheels and the rail. When the track surface is dry, it allows for optimal tire grip, which is essential for controlled deceleration and stopping.

Braking on a well-maintained, dry surface ensures that the braking system can effectively translate the mechanical force into stopping power without losing traction. When the friction is high, the potential for skidding is minimized, making it easier for the train to come to a safe stop.

In contrast, a wet and slippery track reduces the coefficient of friction, increasing the chance of skidding as the train tries to stop. Similarly, applying the brakes too hard can lead to a loss of traction, regardless of track conditions, which increases the risk of skidding. While a fully loaded train may have increased stability due to the weight, it does not guarantee that skidding will not occur. Therefore, dry and polished tracks represent the optimal condition for minimizing the likelihood of skidding during braking.