On rail the largest costs are building and maintaining the rail until you get to very high frequency. For most rail in the US the largest factor in maintenance is weather and so you could run a lot more trains without changing the costs much. You do need to buy more trains, and they will need to be cleaned, but it wouldn't be hard to get enough to people on board to pay those incremental costs. (in the US the bottleneck is often an expensive tunnel that is shared between several not busy lines, each line could itself handle many more trains all day than they have at the peak without changing maintenance costs - but the tunnel is full and it costs too much to build a new one - this is why so many in transit are focusing on construction costs - if we can build a short tunnel we unlock a lot of better transit)