I'd ask "excuse me, is this my session or yours?"

Maybe not, I've never had a rambling T, but I'd like to think that would be my response to one, because it would annoy and frustrate the begeeziz out of me...

How about discussing with T what a polite STFU would look like? It could be a code word or a signal, like when a preschool teacher puts her finger on her lips and (ideally) the class follows suit and falls silent...

Just an idea.