You don’t need to replace everything. You can fix things. It’s probably billions still, but I still say the money should be spent. Even if it’s hundreds of billions over the new couple decades, spend the money, fix the subways. Start with essentials. Then continue maintenance while doing simultaneous improvements.
Let’s be clear, if you don’t spend money you definitely can’t fix the problem. Will only money fix it ? No, but if you need to change management, change expectations, change schedules, do that. All that will cost money as will buying new cars and new signal systems and hiring crews to install the signal systems. What’s needed to fix the system is a lot of changes and they will all cost money. But the reason the changes haven’t happened is due to money. Boston’s system has certainly suffered from lack of money. Sure they have billions, but it isn’t enough.
If you don’t have enough maintenance people to do the work, that is still a money problem. If you pay more for that job, more people will want to do that job, and then you can hire people to staff the positions correctly. It might take a few years for the next generation to come into the workforce, but I believe it can be done.