Japan returns lost property at a rate that startles people. Something left on a train usually reaches the station office, then a central lost-and-found, and it is usually still there days later. What you need is the vocabulary to describe it and ask.

Say where and when you lost it, then what it looks like. Station staff will phone along the line. For items left in taxis, the receipt has the company and vehicle number on it, which is why keeping it matters. The national lost property line for rail is 0570-550-142.