They drove me crazy. Who? Everyone.
Make a reservation for the train they said, otherwise you'll be shut out. Ok, so we made reservations.
When we departed Tokyo for Kyoto (pronounced Coy o toe), we booked a 0930 ish train, as the 0900 train was 'sold out'. Hogwash!
What I found out was that you 'CAN' show up for an earlier train. You may even get a seat.
Some train have only reserved seating. But many trains have a couple of cars with 'unreserved' seating. On all of our trips, where we decided to depart earlier/later than planned, we just showed up and got seats.
Example...Lets say that car 1,2 and 5 are unreserved cars. When you get to the platform you line up by car # (which is marked very clearly on the ground). You do this whether you have a res or not.
If you don't have a seat, you might line up at car #2. The Japanese are very organized like this. Being the first in line, rather than the 16th, would insure a seat...otherwise you'll stand.
I guess my point is that we could have shown up at the station earlier than our original reservation, if I would have known about 'how to obtain a seat in a non-reserved car.'
I think this bit of advice is new to Flyertalker.

Hopefully you'll be able to save time in the future if your travel plans warrant a change.
Food and beverages are sold on the train by hostess'. It's very cute.
And it's pronounced Shin kin SEN, as in Wisconsin.
Dan