Left a bag on the train

I left a backpack on the Chuo line train last night around midnight from Tokyo to Shinjuku. It has a camera, lenses, cash, and sunglasses. Is there a way to retrieve the bag?

Where are you now? Can you go to Shinjuku station right away?

You can contact the JR lost & found office with the description of the item (like the color/approximate size and material of the bag, and the contents, just like you've described here - but don't leave any more details here as someone can pretend to be you), and leave a phone number where you can be reached. Check back once a day or so as long as you are near by, or get their phone number and call.

For a starting point, their English language help center:
https://www.jreast.co.jp/e/customer_support/infoline.html

Hi,

I'm still in Shinjuku. However, I am leaving tomorrow night. I did visit the lost and found at the train station and contacted the JR East infoline as well. As of 5PM today they have not found it yet. They suggested I file a police report, which I did as well.

I'm hoping they find it tomorrow morning before my flight.

Hi,

I am back in the USA but I left my friend's contact info. My friend lives in Japan. As of today, which is two days after I left it on the train, the bag has not been found yet. How long does it usually take for an item to be found?

If it is turned in by someone who found it, that local station might keep it for a couple of days, then they turn it over to their central location for storage; once it is recorded on JR's lost & found database, if your friend calls to ask with a detailed description of the lost item, they might be able to match it. If that doesn't happen, after some days (I am not sure exactly how long) they will turn it into the local police's lost & found. Ask your friend to call in.