Today is March 21, Spring Equinox.  There are blackouts scheduled but 
they may not be necessary it being a holiday.  If they do happen, 
several rail lines will be closed.

Odakyu: Hadano - Odawara
Seibu: Kotesashi - Seibu-Chichibu
Tobu: Isezaki Line north of Ohta, Nikko Line north of Minami-Kurihashi, 
Kinugawa Line and Utsunomiya Line.

These are all scheduled for the afternoon.  TEPCO will announce whether 
power will be provided by Noon.  Odakyu and Tobu say they will keep all 
lines open if the blackout is avoided.

JR East plans closures for Sagami Line, Oume Line, Itsukaichi Line, 
Hachiko Line and Nikko Line in the afternoon.

Exact time frames of line closures available here:
http://www46.atwiki.jp/earthquakeinfo_en/pages/15.html#id_642f5038
(Nicer index.  For some lines there is information on future schedules.)

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Part of the JR Sagami Line (Chigasaki - Samukawa) re-opened today, as 
well as part of the JR Ryoumou Line (Shin-Maebashi - Maebashi - Isezaki).