I have overnight train tickets for a few nights... In some cases I am in car # 76 or #412. How are they sequenced on a train? There can't be 400+ cars. How do I know where to look on a long train? Or where to stand on the platform?

there is often no rhyme or reason to train car numbers IME

usually on the platform there is a Train Composition board that will clearly show where on the platform or at least where on the train your car number is so you can guess where to stand.

so check your car number with the diagram.

each train car on overnight trains will clearly carry the car number on the outside of it.

First: WHERE are you planning to take the train, which countries? the rules and organisation depends on this!

>There can't be 400+ cars.

No, usually it is 10-20 cars.

>How do I know where to look on a long train? Or where to stand on the platform?

Most platforms in Western Europe (where long-distance trains stop) have coach plans for the trains stopping at the respective platforms. So if you arrive a few minutes earlier you have enough time to sort this out.
In worst case, just enter ANYWHERE and go through the cars until you reach the right one (in doubt ask the conductor). It can be a pain with luggage but still better than miss the train.

We did three overnight trains while we were in Europe last month. In each case the train left from our station so it was actually in the station for at least 1/2 hour before it left. When looking for the train carriage - the number in usually on a piece of paper on the door of the carriage. We usualy sat toward one end and waited for the train to arrive and then walked until we saw the carriage number. We took overnights from Munich to Florence. Venice to Paris and Paris to Munich. I hope this helps

>In each case the train left from our station so it was actually in the station for at least 1/2 hour before it left.

I understand your caution, but 1/2 h is way too much. Be there 10 min before the train arrives and you will be more than fine.

As to those "composition" posters...I noticed here in France that in some stations they are now electronic.

And sometimes the car numbers are posted on pieces of paper stuck to the car door windows and on other pieces of equipment there are electronic number signs on the side of the car near the door.

Regarding arriving 10 minutes before the train-- that may be good or bad. We have often arrived earlier than that for long haul trains and managed to board a train that was starting at that station and stow all our luggage at the end of the car. Then we sat and watched the last minute arrivals get on board with major luggage and struggle to get it above them or even find any place at all as the bins at the end of the cars quickly fill up. The origin of TGV in France (particularly Paris) trains, and Eurostar in Italy trains (particularly Rome or Venice) are good examples of this.

bostraveler,
There is a good intro to train travel at http://tinyurl.com/eym5b.