The difficulty in building rail lines on existing freeways is similar and perhaps more complex than adding carpool lanes to existing freeways. A California Department of Transportation practice does not allow the conversion of mixed flow freeway lanes to carpool lanes (and/or transit) without the addition of new lanes to replace any lost automobile capacity. And widening freeways for transit use is tremendously expensive because of the high cost to acquire freeway adjacent real estate, the cost to reconstruct freeway bridges, over crossings, ramps and other facilities.
If transit was constructed on freeways without the requirement to replace lost traffic lanes, the transit stations on freeways would still be perceived by many to be less safe and more difficult to use than stations built in more populated areas. Because of their surroundings, freeway transit stations include high noise levels from adjoining high-speed vehicles, as well as airborne dirt and pollution. Due to the need to construct pedestrian overpasses, bridges, stairways and elevators/escalators from remote parking lots and bus stops, these stations can be challenging to access. Freeway stations are generally not located close enough to major job centers or other destinations to be walk able and, therefore, multiple transfers from trains to buses are usually required.
So although Metro weighs the use of freeway transit projects in locations where it is feasible to do so, freeway transit projects are often more expensive and less desirable than other choices.