eneral must accelerate and decelerate slowly because the rails and wheels are smooth metal which have low coefficients of friction. The brakes on a single car can be applied but they will still take considerable time to stop that car. If the wheels lock up then it will likely loose all traction and take even longer to stop. So the stopping power of a single cars brakes are already insufficient in an emergency situation. Now add tens or hundreds of additional cars and you end up with a compounded problem and therefor the train will take much longer to stop.

COF is independent of surface area. in fact, in cases where both surfaces are so hard is where the simplistic cof approximation works better. In a car is surface dependant because of pressure distribution, microsurface roughness and temperatures and thousands of messy phisics. But in a steel vs steel situation, surface area is a very very minor factor