Miami is a relatively new city, founded in 1896. In 1980, Miami began construction on its first 21-mile heavy rail line that opened in 1985. The expansion of the metro system was a part of the late 70s investment in transit across the US, as seen in DC, SF, and Atlanta.
Metrorail is now a 24 mile long system that transports roughly 60,000 people a day. However, there are gaps in the system that miss key neighborhoods and destinations. The transit fleet started car replacement in 2017, and the new fleet is almost completely deployed.
In addition to heavy rail, Miami is home to quite possibly the most effective people mover in the United States - the Metromover! Although it is only 3.5 miles in length, it transports 27,000 people per day throughout downtown Miami, with connections to important locations.
Miami also has its own commuter rail, TriRail, that opened in 1989. Started as a temporary service, it now stretches 70 miles and transports 14,000 people per day. However, it misses some key locations in west-Miami, and does not serve downtown directly (although it connects)
However, Miami is home to exciting new rail developments. In 2018, Brightline (the only private passenger rail system in the US) began operation with services to West Palm Beach. Trains travel 79 MPH and will reach Orlando and other high-priority destinations by 2022.