  A growing body of evidence points to critical vulnerabilities of social
media, such as the emergence of partisan echo chambers and the viral spread of
misinformation. We show that these vulnerabilities are amplified by abusive
behaviors associated with so-called "follow trains" on Twitter, in which long
lists of like-minded accounts are mentioned for others to follow. We present
the first systematic analysis of a large U.S. hyper-partisan train network. We
observe an artificial inflation of influence: accounts heavily promoted by
follow trains profit from a median six-fold increase in daily follower growth.
This catalyzes the formation of highly clustered echo chambers, hierarchically
organized around a dense core of active accounts. Train accounts also engage in
other behaviors that violate platform policies: we find evidence of activity by
inauthentic automated accounts and abnormal content deletion, as well as
amplification of toxic content from low-credibility and conspiratorial sources.
Some train accounts have been active for years, suggesting that platforms need
to pay greater attention to this kind of abuse.
