I wasn't talking about bridges, I was referring to transport. I wonder
why there isn't one Smart Card for the greater New York area, similar to
what we have here in London.

As I've noted before, Metrocard is accepted by four agencies, five if
you count PATH and the JFK Airtrain separately. Given that it appears
to be technically limited to a fixed fare per swipe, that's all the
places where it makes sense.

A smartcard would presumably also work on Metro-North and LIRR, which
are also the MTA, with the remaining question being NJ Transit.

Since New Jersey is, in the words of B. Franklin, a keg tapped at both
ends, NJ Transit also has to consider interoperation with SEPTA and
PATCO which operate transit in the Philadephia area, even though
there's no need for New York and Philadelphia payments to interoperate
directly.

Extra bonus confusion: NJ Transit is experimenting with tap payment
using contactless credit cards on some bus lines. In Connecticut,
Metro-North runs commuter trains west of New Haven, the state runs
commuter trains east of New Haven, with joint fares, and also joint
fares with some CT Transit buses. On a few routes, Amtrak (the long
distance train agency) will accept local commuter tickets.

R's,
John