My old commute into London was £3,660 per year - standard class. Pretty close to 20% of my net at the time. Plenty of people do it because big cities are where the opportunities are.
And that's not necessarily out of line. Where I live outside Boston which is within a (long--about 90 minute) commute into the city, commuter rail passes (including subway/bus) + parking at commuter rail station would run you about $6,000 per year.
But the point of comparison most people are making isn't with the infamously public-transport-averse USA, it's with countries in Europe where rail travel is state run and heavily subsidised (and the house price savings from living further up the line are smaller)
My old commute into London was £3,660 per year - standard class. Pretty close to 20% of my net at the time. Plenty of people do it because big cities are where the opportunities are.