The illusion of paper trails
Some voting machines such as the ES&S Model 100 use an optical scanner to scan the voter's paper ballot. If the paper ballot is kept, there is a paper trail.
The Premier AccuVote-TSX voting machine works differently:
"If the TSX is equipped with the voter-verifiable paper trail, the printer tape is located to the right of the touch screen, under transparent plastic. Voting takes place as described above however, at the conclusion of voting, a paper ballot is printed and displayed in the Accuview housing so that the voter can verify their selections before the ballot is deposited into a container within the printer module to await retrieval by poll workers."
Meaning the validity of the paper trail relies on the voter diligently checking the printout before it's sent on its way.
BTW, hacking the election doesn't require an attack at scale. The Republican candidate can win the election by winning in 5 swing states: Florida, Pennsylvania, Ohio, Michigan and North Carolina because they have more than the 95 electoral college votes a Republican candidate needs (assuming he doesn't lose in one of the states that's been won by the Republican candidate for decades). Within those states an attack can be focused. In Ohio, for example, compromise the vote in 14 of the 88 counties and you own the result.