Marc Brunel's Thames tunnel was originally built for road traffic but happens to be more or less just the?

Question

right size to fit a standard guage train. Which is what it is now used for as part of London Underground's East London Line. This fact seems to convenient to not have been planned. But I can find no evidence that it was planned to accept trains when it was being designed. does anyone here have information regarding this particular aspect of the Thames tunnel

Answer 1

Not 100% sure but this might shed some light on it...