Maintenance Crises are Emergent

Fascinating element of visiting the transit museum in NYC was learning that maintenance crises were a recurring problem. Politicians suppressed any increase in the transit fare, starving operators of revenue, resulting in deferred maintenance, until some tragic incident generated sufficient public outcry to support raising additional funding by raising fares. The initial five cent fare was just too low to support safe operations and raising it at a rate sufficient to keep up with actual costs (and inflation) has been a constant struggle. 

Source: Wikipedia, via Reddit r/nyc: NYC Subway Fare over last 120 years adjusted for inflation

Emblematic of infrastructure in America, really. We subsidize use by underpricing user costs. 

Network Expansion Limits

The special magic of the network effect: Adding node C to path AB not only makes journeys from B to C, but also A to C. So there is an inherent non-linear scaling to adding links. However, while each link adds more because of the network effect, but each link is less-good than the last, because you always build the best links first. So at some point, any network faces declining marginal returns on expansion. 

Further, in any infrastructure network, expansion is perilous, because infrastructure has non-linear maintenance, and those maintenance costs are end-loaded--cheap initially, but with a ruinously expensive (and unavoidable) balloon payment for reconstruction at the end of their useful life*.

*Rail miles, like road miles, are more discrete than things like bridges, so the necessary 'balloon payment' is much smaller, and it's possible to make a lot of low-quality 'patches' that neither work well nor are cost efficient. (If the patches either worked well or were efficient, you'd have built the how system out of 'patches' from the beginning).