Fees shouldn't be based on "which line is busier" (I didn't say that, so why quote it as if I did?), it should be based on what it costs to accommodate the next passenger. If the Yonge line is at capacity, the cost to add another passenger involves somehow making more capacity (bigger stations, bigger trains, better signaling) which involves hundreds of millions of dollars. Conversely, if a route is running under capacity the cost to add another passenger is basically zero. This isn't exactly free market radicalism, it is just logical that services should have an underlying relationship with their costs. Many people here, including myself, would argue that the best way to deal with road congestion is via some kind of pricing mechanism. The underlying logic is the same, if our roads are at capacity the cost of adding another car is the cost of adding more roads (which is typically expensive).