The drag is air against the whole body of the train, so you need vacuum everywhere.

Assuming that you could build such a big vacuum there would be safety concerns. What if there’s an accident in the tube? Does everyone in the train depressurize and die? Assuming people can survive and get out of the train car, now they’re in a tube that’s 100 miles long. How can you build emergency exits in a system designed to be as airtight as possible?