4+6-car train.

[Sascha]

Sometimes they have Train descriptions that say 4+6 like E233-0 series: 10-car (also 4+6-car ) sets used on the Chūō Line (Rapid), Ōme, and Itsukaichi lines since 26 December 2006. Why doesn't it just say 10 car set? 

[kvp]

Mostly because the train consists of multiple subsets. In some cases they even split along the route and continue towards different destinations. (in case of the Ome line only the 4 car subset goes all the way to the end of the branch, but gets rejoined with the other 6 cars and the 10 car combined set goes back to Tokyo station as a Chuo line train) Older consists on the Chuo line had 5+5 formations. Also some newer JRC sets are formed fully from 2 and 3 car subsets (allowing 2, 3, 4, 5, 6, 7, 8, 9, 10, 11 and 12 car set to be built on the fly although not all combinations are in daily use)

[bikkuri bahn]

At Otsuki Station.  A four car e233 set couples on to continue on to Tokyo.

 

At Haijima Station.  A similar operation.

 

[Socimi]

They are coupled sets that split for services on different branchlines, but with a common mainline route.

 

Specifically, the Chuo Line 4 car sets are split at Hajima station, 4 car sets are for the Ome Line to Okutama and 6 car sets are for the Itsukaichi line to Musashi-Itsukaichi.

 

On return trips, they are coupled also at Hajima station and they continue to Tachikawa, when they join the Chuo Line for Tokyo and Shinjuku.

 

Similar operations with coupled sets in the Greater Tokyo Area are on the Shonan-Shinjuku Line (10+5 car sets of either the E231-1000 or the E233-3000 series) that are split at Koganei station for the Ustunomiya-Yokosuka Line route and  at Kagohara for the Takasaki-Tokaido Line route; on the Joban Line (also 10+5 car sets of either the E231 or the E531 series) that split at Toride (the 5 car set continues onto the Narita line) and on the Tokaido Main Line (again 10+5 car sets of either the E231-1000 or the E233-3000 series).