* Inertia is directly proportional to mass. Inertia is the tendency of an object to resist changes in its motion. The more massive an object is, the more inertia it has.
* Trains are significantly more massive than cars. A typical train can weigh hundreds of tons, while a car might weigh a few tons. This huge difference in mass translates directly to a huge difference in inertia.
Think of it this way:
Imagine trying to push a small car and then trying to push a massive train. You'd find it much easier to get the car moving and to stop it once it's in motion. The train would be much harder to move and much harder to stop because of its much greater inertia.
In summary: Even though the train and car are traveling at the same speed, the train's greater mass gives it much more inertia, making it harder to change its motion.
