Anywhooo ~ According to TfL there are 534 trains running at 0900 hrs on any given week day and this is the maximum. It tails off during the day and at weekends and curiously doesn't reach that peak again in the evening rush hour.
So 30 tonnes x 534 trains = 16,020 tonnes
This doesn't include the passengers of course.
That is about the same as 1314 London Double Decker Buses (unladen) which are about 12 tons (UK) & that converts to 12.19256 tonnes each.
(Note some sources prefer to say that a laden London Bus weighs about 15 tons & other say that they weigh a mere 9.5 tons but I guess it all depends on which model of Routemaster you wish to use.)
Now where did I put my anorak?