"Neil Williams" wrote

[snipped]

Paying on exit does, however, have the disadvantage that you must
get
at least a small proportion of people who only realise they don't
have
appropriate means of payment after the debt has been incurred by
completion of the journey, rather than noticing when you pay
beforehand
and therefore being able to address the issue before it's too late.

Which led to the famous song arising from the time when the Boston
MTA raised the subway fare from 10 cents to 15 cents some time in the
1940s (the song was revived in the 1960s) The MTA wasn't immediately
able to modify the turnstiles, so during the interim you had to pay
10 cents to get into the system, and another 5 cents to get out.

The song told of Charlie and the MTA

... he put 10 cents in his pocket, kissed his wife and family, and
went ride on the MTA

Did he ever return, no he never returned, and his fate is still
unlearned
He will ride for ever 'neath the streets of Boston, Charlie on the
MTA

Jeremy Parker