Safety Assurance is a big part of rail transit design but I wouldn't say it is a significant cost. Building and operating rail wouldn't cost half as much if we stripped some of the safety assurance process out of it. Maybe a couple of percentage points of cost for orders of magnitude more fatal accidents. The systems would mostly all still be there, unless you wanted to do something as drastic as removing all interlocking/train protection and rely entirely on drivers and timed signals - basically treat it like a road network - but that would have huge operational impacts - train headways have to be much larger if you can't know what sections have trains in them - as well as being extremely unsafe.

Frankly, these precautions exist for a reason, and it is because many many people died in the early days of rail. Many people who work in rail, particularly in ops/sigs/assurance are proud of how safe rail transport now is.

(I work in Engineering Assurance on rail infrastructure projects).