Oh of course, it's not even in the same league with extremely wasteful highways. My point was just that tracks have limits too and at some point you need to ad

Oh of course, it's not even in the same league with extremely wasteful highways. My point was just that tracks have limits too and at some point you need to add more of them / add more train cars / add new lines. This can be seen in Paris where RER A was built in part to decongest line 1, which then resulted in both being heavily congested (RER A is one the most heavily used commuter rail line in Europe and the whole world). This led to the creation of line 14 to decongest the A and 1 inside of Paris, but it got congested too, so now there's also the RER E to decongest the 1, 14, A and B by taking over some trips between destinations that would have required doing a change 14+A or B+A.