What the officers did (not checking and registering everyone) was illegal, and unfortunately it’s always the traveller that gets to pay for it.
You must go tomorrow morning to the deferred inspections site located at Philadelphia Airport, terminal A West, open Monday-Friday 09:00-12:00, and present your visa in the passport, I-20 form, and the train ticket to the US. They will take the necessary steps to Register your entry in the records.
To get to the airport, take the SEPTA train from central Philadelphia.
Credit:stackoverflow.com‘