1. All of their tickets are valid on a particular trip only.
2. As passengers board, the driver checks them off a passenger manifest.
3. One member of staff concerned with max 80 tickets. No correct ticket - no travel.
3. This still doesn't stop some clever people trying to board with a made up number, or two passengers trying to board (front and back of queue) using the same number and hoping the driver thinks he made a mistake in ticking off the manifest.
Compare this with trains, where only comparatively few tickets are valid on one train only, where staff are not checking each passenger before they board, checking 500 passengers. Not practicable. Too slow a system and too wide open for train use.
The Czech Republic has a dense railway network with fairly frequent services on many routes.
They say:
How I will get the ticket: ticket in PDF format that you can print out on your printer. You can also display the ticket in PDF format during an inspection on the train on your portable electronic device – e.g. on the screen of your laptop. You can also present the ticket to the conductor displayed on your mobile telephone in the form of a displayed PNG image with a QR code. In the case of inland tickets, instead of presenting a printed ticket you can also just communicate the transaction code, which you will find on the bottom right of the ticket, above the graphic code. If you enter your In Karta card number when purchasing an inland ticket in the e-shop, then it is sufficient to present just your In Karta card to the conductor during a ticket inspection on the train.
Now I concede that I'm just going by what's on a web site not from experience, but this suggests that they have made it work.
It might fall over if every passenger insisted on just showing a transaction code, but you can be pretty sure that most people will be able to show their ticket as a printout or on the phone.
This does remind me a bit of Americans arguing how Amtrak couldn't possibly reserve particular seats for passengers because it would be far too complex as unlike planes trains make lots of stops. But the European railway system shows that it would be quite possible.
I agree that the Megabus example isn't very good evidence that this would work for trains. But there was "Megatrain" (and maybe still is?) which I thought worked in the same way so only point 1 above would be relevant.