Why does it take longer for a train to slow down than an automobile?

Why does it take longer for a train to slow down than an automobile?
A train, especially a freight train, is a massively heavy vehicle. The difference between stopping a car and a train could be compared to the difference between stopping a rolling golf ball and a rolling boulder.

Each set of wheels on a train has it's own brakes, but still, because of the mass of the train, it takes considerably longer to slow a train down. Also, there is less friction on train wheels and track than there is between tire treads and roads. If all the wheels of a train were suddenly stopped and locked, the train would still skid for a very considerable distance on the steel wheels and track, ruining both.