Mass transit in most places doesn’t come out ahead even accounting for externalities. The average passenger vehicle emits 0.91 pounds of CO2 per mile. Many estimates of CO2 price it at $30/ton. At that rate, the CO2 externality from driving is 1.4 cents per mile. The cost of CO2 recapture puts a cap on the cost of CO2, and currently it’s $100-$230 per ton. At that price, we are looking at $0.10 per mile.

Typical subsidies for transit are in the $0.50+ per passenger mile range for rail, and $0.80 for bus: https://opportunityurbanism.org/2019/09/transport-costs-and-... (scroll down to the graph).