Here is my proposal for making rail transit cheaper
Rails by themselves are actually quite cheap - anywhere from 1 - 2 million dollars a mile. Monorails are generally cheaper than heavy rail, due to less complicated elevated structures.
Electrification can be anywhere from 5 - 10 million dollars mile for overhead wire and 1 - 5 million dollars mile for third rail.
The biggest costs by far are land acquisition, tunneling, bridges, elevated sections and viaducts. Eliminating at grade crossings and either moving roads underground or rerouting them can also be expensive, but this is rarely done
My solution to this is to use narrow gauge tracks. I would use a 3 foot 6 inch gauge, because this gauge is farely common. 3 feet 6 inches is roughly 25% narrower than standard gauge. This narrowness would enable smaller Bridges as well as tunnels. Narrower gauges also enable using tighter and sharper turns; this makes it ideal for building along freeways. Many suggest the idea of building rail systems along freeways, but freeways often have sharp turns. A narrow gauge railway enable using the curved and sharper rows of freeways, reducing land acquisition cost. Where tunneling is needed you could use smaller tunnels, the New York and Bart Subway tunnels are roughly 17 feet in diameter, the London tube is roughly 12 feet in diameter. Boring company tunnels are roughly 12 feet in diameter, but their utility is limited by the fact that can only use cars and thus have a limited capacity 1000 - 4000 passenger per hour. You could take these tunnels and put some narrow gauge railways in them and exponentially increase their usefulness.
Finally the vehicles would be similar to PCC, albeit with a more up to date design. A PCC car is quite efficient and uses less weight to move a passenger than modern light rail vehicles do. A narrow gauge PCC car seats 47 passengers and weighs 37,990 pounds, this 808 pounds per seated passenger. A Sie