1. That their hourly service really was hourly and that this schedule was consistent every day of the week. I don't expect this to be the case for the first couple of years (in large part because of the expense and unavailability of enough train sets to manage it), but it's a reasonable assumption for a mature service.
2. That it took four hours from departure until the train arrived at the final destination, was turned around, and departed again.
Between these two assumptions, the result was eight train sets with a total of 32 daily trips between Orlando and Miami with the first train leaving at 6am and the last leaving at 10pm. With a 240 mile long route, that runs to a hair over 2.3 million train-miles per year. At Amtrak's FY2011 budget of $78.42 core expenses per train-mile (page A-2.2), that presents a total annual cost of $219.8 million to run the passenger service. This, however, might be considered a high end figure. A British report (Figure 2.11, page 30) indicates that their cost of train service is £10.6 per train-kilometer plus an additional £6.83 for track access charges (which we may lump together) for a total cost, after conversion, of $44.76 per train-mile (although this figure is somewhat normalized for direct comparisons with various European nations rather than being an exact figure). Since Metrolink achieved $55.7 in 2006 (page 5), $60.14 in 2010 dollars, this seems a reasonable figure and we may figure $125 million per year as a low end figure (an additional support may be found in Figure 3.1, page 23 of the 2007-2008 Pacific Surfliner Business Plan which quotes $48.43 as train expenses per train mile, however this may not include infrastructure and does not appear to include other overhead which the British figure does).
Interestingly, although using the British figure as the lower bound, it is entirely possible that the numbers might be even lower. A brief examination of Amtrak's FY2009 budget, the last to contain expense figures for the entire year, indicates that the Piedmont costs as little as
With these figures, the question becomes one of ticket pricing. Referring once again to Amtrak's budget, their average ticket yield of 28.96 cents per passenger mile equates to a $69.50 ticket for Orlando to Miami. This is actually a fairly price competitive fare given a US average fuel efficiency of 23.8 miles per gallon, $4 a gallon gasoline and tolls on the competing roads (although there is a 30 minute slower routing which is free of tolls), especially given the car competitive times of the planned rail service. Assuming this, All Aboard Florida would require 1.8-3.2 million passengers, paying full fare and traveling the whole length, in order to break even. Higher or lower fares, of course, would change the numbers.
Is this a reasonable figure? It does appear to be so. The ridership figures fit comfortably within the potential carrying capacity of the trains (at 500 seats per train, in line with current Surfliner consists, they represent a 30-55% load factor). Both numbers are also significantly lower than the Florida High Speed Rail Authority's Orlando-Miami planning study predicted for a 120+ mile per hour system, although with a longer travel time as well (2015 ridership of 4 million, with a 2011 start, and a travel time of 2:33). Provided that they are able to market themselves well, especially to the younger demographics, they stand a healthy chance of reaching that ridership level.