Much of Baltimore's sewer system was rebuilt anew after the great fire. Because of that, we have one of the most modern systems of the Northeast.

While other cities' systems required massive upgrades in the 50s and 60s to separate their sanitary from their stormwater, we didn't have that issue. Yes, it's poorly recorded by as-builts/GIS, but that's not a huge issue in itself. Even well mapped systems need lots of field verification.

Infrastructure investments are made for 80-140 year lifespans, which means that timing can make a huge difference.