According to the MBTA website:
Bus and Subway transfers are discounted when you use the CharlieCard. With a CharlieTicket, the full fare, plus the surcharge will be deducted from the stored-value on your CharlieTicket when you board the bus or subway.
With a CharlieCard, simply board the first vehicle of your trip and the corresponding fare is deducted. When you transfer, the faregate or farebox will automatically recognize you as a transferring customer, and deduct the additional transfer fee, if any. When transferring from a lower-priced service to a higher-priced service, like Local Bus-to-Subway, the price of your transfer is simply the difference between the two fares ($0.45). When transferring from a higher-priced service to a lower-priced service, like Subway-to-Local Bus, there is no cost to transfer.
Not quite so, remarks our friend. Apparently, your CharlieCard is time-stamped when you first get on the trolley line, and if the time you enter the bus is more than a certain number of minutes afterward, the discount doesn't work. Ditto on the return trip. A bus driver told this passenger that the second ride has to begin within 20 minutes of the first ride.
I don't know if the driver was correct, but -- as locals will tell you -- the trolley ride itself is likely to be more than 20 minutes, and if you have to wait for the bus connection, it might be 40 minutes or more before you actually get on the bus. Ditto on the return.
Apparently you can write to the T and ask for a refund if the transfer discount doesn't register. But that takes a couple of months to process. And, who wants to do that every day?
So, if this is all true, why would the people at the T be so concerned about a transfer having to occur within a certain number of minutes? Are they really worried about people "abusing" the system by, say, stopping for coffee en route to their destination? Do they think someone will hand off his or her card to another passenger at the junction of a rail and bus line and use it later in the day?
And, if it is not true that there is a specific allowable time interval, why doesn't the discount work?