The number of passengers is not the optimum way to look at the issue. A better way is to look at the passenger miles sold vs the available seat miles.
In FY11 Amtrak had 12,530,314,000 seat miles for sale, of which it sold 6,532,250,000 passenger miles for an average load factor of 52.1 per cent. If it had sold 65 per cent of its average capacity, it would have 8,165,312,000 passenger miles. If it had sold 80 per cent of its capacity, which would rival the average load factor for the nation’s airlines, it would have had 10,024,251,000 passenger miles.
The average trip in FY11 was 216.4 miles. If the same ratios held up for the aforementioned expanded capacities, it could carry 37.7 million riders at an average load factor of 65 per cent capacity and 46.3 million riders at an average load factor of 80 per cent of capacity.
On average Amtrak has the scalability to increase its load factors without have to make a significant increase in its capacity. It probably could do so with a relatively low marginal increase in costs. Beyond 80 per cent of existing capacity would require a heavy investment in incremental capacity, which would not be a good idea until Amtrak could demonstrate higher utilization of its existing capacity.
Is there a market (aggregate demand) for an expanded intercity passenger rail market? I doubt it, at least for now, although there could be in the future as congestion, pollution, convenience, improved transit, etc. make passenger rail a more desirable choice. Even the highly praised Acela trains had an average load factor of just a bit over 62 per cent during FY11.
In addition to a lack of demand to fill its trains, Amtrak would have a major challenge on its hands if it tried to expand its operations over its partner railroads. Witness the failed attempt to get the UP to hoist the Texas Eagle on a daily basis between San Antonio and Los Angeles.