Yeah but I dont agree that 30,000 is an absolute minimum. I prefer an estimate like $100,000 per rider and $100 million per mile construction would require 1000 riders per mile. Busier lines can cost more to construct and lines that are cheaper to construct won’t need as many riders so it is a handy little rule of thumb to go by.

If you want a similar sized city Portland is the closest to Pittsburgh. They spent $1.5 billion with 50% federal and 25 % state funding for 7.3 miles with a projected ridership of 17,000 puts it at $88,000 per projected rider. Sadly that lines projections were two high and actual ridership has come out to be around $200,000 per rider. Some agencies are better than others at estimating and the San Diego line had near perfect estimates.

Here is a great article with a lot of data. It discusses heavy rail in big cities first which is obviously not comparable to Pittsburgh but later in it gets into light rail in other cities

https://www.bloomberg.com/news/articles/2018-01-26/the-u-s-gets-less-subway-for-its-money-than-its-peers