Metra may soon be totally cashless on trains according to the SunTimes.
Right now, you can still purchase a ticket with the conductor for a $5 surcharge, but that may soon change. They want to speed up the ticketing process and limit robberies by stopping conductors from carrying cash.
One possible solution would be to add on board machines that would accept cash.
I use my Ventra app, but sometimes its glitchy and doesn't work the exact moment I need to show my paid fare (which is super stressful).