The issue isn't the cards - it's the crummy readers on the T fare machines. I don't think they've EVER once read ANY of my cards correctly on the first swipe. Usually it takes 3-4 attempts (which is annoying, because the machine automatically cancels the transaction after the 2nd or 3rd error). In the past it was always "BANK CARD NOT READABLE", recently I've sometimes gotten "BANK CARD TYPE NOT SUPPORTED" as well.
They probably don't clean the card slots. Hands contain oils which are transferred to cards when held, and is transferred to the card reader when swiped. People who use hand lotion also add to the gunk buildup. It seems insignificant, but after years of use, it will build up to the point where it will not read reliably.
Where I work, they clean the pin pad magstripe readers and chip slots weekly with a cleaning card (a textured credit card sized card soaked in alcohol) and the cleaning card comes out black.
Add to the above, the fact that so many (i.e., damn near all) of the FVMs are in pretty dirty locations (whether inside stations or close to outside elements) and it's probably a miracle as many work as they do!
Wondering if I'll see the Haverhill double-tracking finished before I retire... Photo: Melbourne W7 No. 1019 on Route 78, Bridge & Church Streets, Richmond, Victoria. 10/21/2010
The magnetic strip can become demagnetized by exposure to a magnetic field (wonder what a ringing cell phone next to the credit card does), or mechanical damage from carrying the credit card loose in your pocket. I sometimes have trouble with my ATM card at the bank teller, which is probably the oldest card that I carry.
The closest magnetic thing that may interfere with the card is... well, other cards! I've noticed that if I had a Link Pass on a ticket, that it wouldn't read right away if I kept it in my wallet (the whole in and out deal at the farebox/gate), but wouldn't have this issue if I kept it in a different pocket. Hotel keys have the same issue, they can get demagnetized by credit cards.
Credit cards are actually quite resilient to such interference though, and most of the time, if my cards stop working, it's because of physical wear on the magstripe.