Plans had long been floated for an extension from Dublin to Livermore, but the most recent proposal was rejected by the BART board in 2018. Other plans have included an extension to Hercules, a line along the Interstate Highway 680 corridor, and a fourth set of rail tracks through Oakland. At least four infill stations such as Irvington and Calaveras on existing lines have been proposed. With the Transbay Tube nearing capacity, long-range plans included a new four-bore Transbay Tube beneath San Francisco Bay that would run parallel and south of the existing tunnel and emerge at the Transbay Transit Terminal to connect to Caltrain and the future California High-Speed Rail system. The four-bore tunnel would provide two tunnels for BART and two tunnels for conventional/high-speed rail. The BART system and conventional U.S. rail use different and incompatible rail gauges and different loading gauges. In 2018, BART announced that a feasibility study for installing a second transbay crossing would commence the following year. By 2019, the Capitol Corridor Joint Powers Authority had joined with BART to study a multi-modal crossing, which could also allow "Capitol Corridor" and "San Joaquin" routes to serve San Francisco directly.