The San Diego Trolley is the system that started the modern 'Light Rail System' craze in the United States when (what is now referred to as) the Blue Line opened from downtown to San Ysidro/International Border in July 1981 along a sparsely used freight ROW (which still sees minimal freight service). The terminus in San Ysidro is just steps from Mexico and the busiest land border in the world. The system has expanded some since opening day to three lines and today it is both the third most ridden and third longest modern light rail system in the United States (locally always called Trolley).
Until all were retired in January 2015 (with some of the cars exported to a new trolley line in Mendoza, Argentina), San Diego was the only U.S. operator of U2s, and the only North American operator of low-floor U2s (Edmonton, 1978 and Calgary, May 1981 use the high-floor variant). The original blue line stations were extremely cheaply built with ground-level boarding and no genuine platforms. These stations have all since been extensively renovated and rebuilt to allow the use of newer S70 cars which require 8 inch high platforms, newer stations were built with six inch high platforms, and have also had two more inches of height done. Until station renovations were done from 2011 to 2015 and low-floor S70 trains were introduced into operation, Orange and Blue Line trains ran using exclusively stations with side platforms and slow on-board wheelchair lifts (similar to those on a bus) for ADA access, with a wheelchair boarding delaying an entire train.
The newer Green Line, which entered operations in 2005 has always run mixed consist fleets of low-floor S70s and SD100 (the intermediate trolley types, used on all 3 lines), with all stations on its original routing between Old Town and Santee newly built or renovated to have 8 inch high platforms for level wheelchair boarding on the S70 service. See this diagram for the 2005 to 2012 service pattern. On September 2, 2012, The retrofitting of all stations between Old Town and 12th & Imperial Transit Center to allow Siemens S70 Avanto operations, resulted in nearly the current service pattern with the Green Line extended to Downtown and the Blue line cut back to America Plaza until the extension to UC San Diego opened in 2021. The Orange Line originally terminated at Santa Fe Depot but was cut-back first to America Plaza on June 11, 2017 and then further back 2 blocks to the newly built Courthouse Station, which is simply a newly built 3rd track and platform to the south of the existing Blue Line tracks when this station opened on April 29, 2018.
Base train service runs every 15 minutes on all 3 Lines with the exception of 7.5 minute service during weekdays on the ultra busy Blue Line (only between San Ysidro and America Plaza, service to UTC is every 15 minutes), and only half-hourly service on the Orange and Green Lines on Weekend mornings (was half-hourly all day on Sundays in early 2010s).