5% Set-off The design of the BorderTransportationSystem is such that it uses the finest component and should last for many decades, but this is a High-Speed system in a very harsh environment and things will wear out and need repair of replacement.  The suggestion is that 5% of each dollar collected by set aside and invested in government secured bonds. 5% over 20 years will cover the existing cost, but we have to add compounded interest over the 20 years which will be approximately 2.5 x the current value. Thus if an item cost $1.00 now, it will cost $1.80 in 20-years, and therefore, the 5% would have to be about 10% in order to pay for the replacement in 20-years. But, if invested in a 6 or more % bond, then the 5% should cover the replacement cost.  Index of ... Money Generation section