The Pakistani state is not poor because of orange line metro. coal power projects or dams but because of lack of tax collection to effectively run it. This is why we had circular debt issues and what not.
Btw most metro system around the world are subsidized. Thats a whole new debate but long story short, it's not meant to be seen as a profit or a loss. I highly doubt the orange line cost $2.6B but if we assume it to be true, it is justified because Pakistan has no experience in building metro rail systems. it had to import everything from professionals to construction equipments to rail equipment as tiny as screws. When it became operationalized, they estimated 200,000 people would use it per day. Today that figure has increased.