When you are in Japan, especially in Tokyo metropolitan area, and waiting for a train at the platform, you may see a train which does not open its doors when stopping at your station.
These trains can be three types:
1) Not in service - The train is on the way to the rail yard or to different station without carrying any passenger.
2) Operation stop - The train stopped due to timetable reason or train meet, due to single track. Usually it happens for limited express or other fast trains.
3) Test run - New trains which are manufactured at train factories must undergo several test runs before entering the services.
Test run is common and frequent in Tokyo metropolitan area due to many trains run on multiple railway companies' tracks. For example, trains on Tokyo Metro's Fukutoshin line will run also on Tobu Tojo line, Seibu Yurakucho and Ikebukuro lines and Tokyu Toyoko line. Which means test runs must be done on all lines.