The earliest U.S. antitrust laws were adopted after technological changes — most importantly, the development of a national railway network — made the U.S. political union a single economic market, according to the authors.
Link
http://econpapers.repec.org/scripts/redir.pl?u=http%3A%2F%2Fwww.egi.ua.pt%2Fwp_economia%2FWP27-StephenMartin.pdf;h=repec:ave:wpaper:272005