You can indicate your destination at the entry turnstile to the platform and it will tell you two things: (a) the car you should board, and (b) the car you should exit from at the destination station. Most times, (a) and (b) are same. Some times, (a) and (b) can be different – say when you have more cars than then length of the platform supports, then some cars will hang off the platform length and hence their doors won't open. If you were assigned to board such a car, you have to make way to another nearby assigned car whose door can open, which is assigned to you so that everyone won't crowd the same last car aligned to the platform.