Train..

It remains unclear how the documents arrived at Biden's home, but as the president noted in a speech, he typically frequented Amtrak, taking the train from Washington, D.C. to his home in Delaware almost daily. 

There is speculation that Joe Biden may have carried the classified documents during such a trip and while he had a security detail during his time as vice president.

Biden's Secret Service protection as the vice president continued for only six months after he left office, and was only reinstated when he became the Democratic frontrunner during the 2020 presidential campaign. It is unclear if Biden – or any documents he may have carried – were protected by any security personnel from mid-2017 until 2020. 

In all, federal authorities have received between 25 and 30 documents marked classified from the office space and Biden's Wilmington home.

Members of the Senate Intelligence Committee – including those of Biden's own party – have expressed frustration at the lack of transparency from the White House regarding the documents.

Since memory lapsed for years, did anything was accidentally left in the train?   

Just a thought.