40 lbs is just a minimum..... I think it could do a lot more with a heavy duty rack made of square tube... 1/8" x 1.5" inch flat bar works good, I used it on my bike that I built last year.
what kind of hills do you have? we have bad ones here, like no way to pedal some of them.
the moulton bikes look very ligth weight, probably very good for the bus.