Which coach you are seated in will be determined at least partly by your destination. Certain cars are used for passengers who get on and off in the wee hours so as not to disturb people sleeping in other cars.
Other factors will include how many people are aboard the train, and how many attendants there are. With recent cutbacks, attendants will likely be putting more people into a given car so they will have less territory to watch over. A year ago I was on a train with two empty coaches so the attendant didn't have to cover them all by himself.
FYI, the attendant or conductor will probably take your ticket after you board then issue you a tag placed above your seat, which indicates your destination. That tag indicates your seat is taken. Do not remove it or you could lose your seat while you are away. If you wish to move to another seat, ask the attendant. If he/she OKs the move be sure to take the tag with you to your new seat.
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Trust God, love your neighbor, and never mistake opinion for truth.
-Mr. Toy