During the G20 Osaka Summit locker services will not be available in certain railway stations.

The G20 Osaka Summit will be held on June 28 and 29, 2019.
During the summit, including several days before and after the meeting, locker services will be unable at certain stations to strengthen the security of the following railway stations throughout Japan.
We recommend that you use luggage storage service at your accommodation.

For the detailed information, please see the following link.
https://asset.japan.travel/image/upload/v1561598681/pdf/G20_Locker_info.pdf

Please contact each railway company about the details.
Main Contact
East Japan Railway Company
https://www.jreast.co.jp/e/customer_support/infoline.html
Central Japan Railway Company
https://global.jr-central.co.jp/en/
West Japan Railway Company
https://www.westjr.co.jp/global/en/pdf/g20osaka.pdf
Kyushu Railway Company
http://www.jrkyushu.co.jp/
TOBU RAILWAY CO.,LTD.
+81-3-5962-0102
SEIBU HOLDINGS INC.
+81-4-2996-2888
Keisei Electric Railway Co.,Ltd.
http://www.keisei.co.jp/keisei/tetudou/skyliner/jp/index.php
Keio Corporation
https://www.keio.co.jp/english/index.html
Odakyu Electric Railway Co.,Ltd.
+81-3-3481-0066
TOKYU CORPORATION
+81-3-3477‐0109
Keikyu Corporation
https://www.keikyu.co.jp/company/worldwide/index.html
Tokyo Metro
https://www.tokyometro.jp/lang_en/support/
Kintetsu Railway Co.,Ltd.
https://www.kintetsu.co.jp/foreign/english/index.html
Nankai Electric Railway Co.,Ltd.
https://www.howto-osaka.com/en/traffic/
Keihan Electric Railway Co.,Ltd.
https://www.keihan.co.jp/travel/en/
Hankyu Corporation
http://www.hankyu.co.jp/global/en/
HANSHIN ELECTRIC RAILWAY CO.,LTD.
https://rail.hanshin.co.jp/
Nishi-Nippon Railroad Co.,Ltd.
+81-92-303-3333
Bureau of Transportation , Tokyo Metropolitan Government.
+81-3-3816-5711
Osaka Metro (Osaka Metoro Services co.,Ltd.)
https://www.osakametro.co.jp/
Kyoto Municipal Transportation Bureau
https://www.city.kyoto.lg.jp/kotsu/
Fukuoka City Subway
https://subway.city.fukuoka.lg.jp/eng/index.php
OSAKA MONORAIL CO.,LTD.
http://www.osaka-monorail.co.jp/jpn/company/worldwide.html