ROME—Italy’s government vowed to crack down on demonstrators threatening to block train tracks throughout the country Wednesday as a rule requiring COVID-19 tests or vaccines to use public transportation for long-distance domestic travel took effect. In a bid to rein in the transmission of infections as Italians returned from summer vacations, the government announced weeks ago that passengers must show a so-called “Green Pass” to board domestic flights, trains, and buses traveling between regions. The passes certify the holder received at least one vaccine dose more than 15 days prior, tested negative in the past 48 hours, or recovered from COVID-19 in the previous six months. The requirement also applies to sea travel, although some ferries are exempt, such as ones serving tiny islands with no other connections to the mainland and those used by commuters between Sicily and the southern tip of the mainland in Calabria. Local buses, trams, …