One our way down into Porto this morning for our train out to Régua, we were nabbed by the transit cops.

The Porto Metro system, like many these days, works on the honour system — you buy and validate your ticket on your own, and there are no gates to pass through, you just walk on. The check against abuse is a team of roving ticket checkers who carry hand-held computers that can check to see if the ticket you bought (assuming you bought a ticket in the first place!) is valid.

Our tickets weren’t valid.

As it happens, we got nabbed just as our stop was coming up, so when we arrived at Trindade station we were accompanied by the Metro Cops. They were actually quite pleasant — no shackles or take-downs, etc. — and when they realized we were non-Portuguese, the gloves came on in full and they were super-nice to us.

The error of our ways was simple: we’d been working under the assumption that when you purchased a ticket, you determined the number of “zones” you needed to purchase by counting the number of zone boundaries you were going to cross. We started in C3, and were traveling through C2 to C1, so always reasoned that this would require a two zone (or “Z2”) ticket.

It doesn’t.

You count the total number of zones that you’ll travel from, through and into to calculate your ticket needs: so a trip from C3 through C2 to C1 is a three zone ticket.

Oops.

Fortunately we were taken pity on (we didn’t mention a week and a half of previous offenses), and let off with a simple purchase of the additional zones required.

There was a subsidiary issue of needing a ticket for Oliver too — ignorance again — but that again was let off with a simple ticket purchase.

In our bleary morning coffee-less fog, this all seemed a little bit overwhelming, but as the day wore on we came to appreciate how lucky we were to encounter such friendly enforcers. Had the situation gone differently, we might have spent the day cooling our heels in Transit Prison rather than floating down the Douro.

Lesson learned.