Anyone who tells you that Amtrak/Airlines/Hotels/Car Rental companies don't overbook either is very trusting or very naiive. With hotels (because it's what I know when it comes to this topic) the projection is that about 5% of your rooms that are due to check in on a given day will not show (for one reason or another). So when you're forecasting how many rooms you want to reserve your number will vary. There have been many a day when I will walk in oversold by 5-6 rooms and have 5-6 to sell by the end of the night due to cancellations. Sometimes your forecast is right. Sometimes it's dead wrong. But that's forecasting for you.
In the rental car industry, the fleetwide average is about 12%. It ranges from a high of 15% for small cars to a low of 7% for large SUVs and vans (my theory on that is that compact car bookers are more likely to rate-shop, make multiple bookings, and then forget to cancel the ones they don't need, while minivan bookers are pretty tied to that minivan because they
need the room and they're not as price-sensitive).
It doesn't help that rental car reservations are generally not guaranteed by a form of payment, so there is nothing for the customer to lose by no-showing (no cancellation penalties like having to pay for the first day like a hotel would do).
Rental car companies therefore plan on overbooking by about 10%, since sitting cars lose money. They overbook more heavily on small cars than large ones, too, for a couple of reasons: one, they can always give the customer a free upgrade to a larger vehicle (whereas if you oversell on a Suburban, what else can you put them into?), and two, it's a way to continue to take reservations from price-sensitive customers (many people ask solely for an economy car, and if the company's sold out on economies, they'll check elsewhere before they check rates on larger vehicles (even if one company's fullsize is cheaper than another company's economy!). However, such practices are now starting to backfire with high gas prices--customers who used to be pleased to get a "free upgrade" to a nice fullsize or SUV are now demanding that the rental company magically snap their fingers and make a Chevy Aveo appear out of thin air, and barring that, paying for the additional gas that the larger vehicle guzzled.
Everyone has limits though. There does have to be some point when you shut the system down and say no more. So even if there are cancellations a day or two in advance its possible Amtrak won't sell those tickets again because that's part of the forecast.
Rental car companies do have a small advantage over hotels in that the availability is a bit more fluid. Rental car availability is not locked in at the end of the night--a wait of even just a few minutes results in more cars being returned, cleaned, and ready to rent. So a rental car company can push the margin a bit more than a hotel can--it will just result in some customers having to wait a few minutes (hopefully not too much more) for a car to become available (whereas a hotel has to tell a reserved guest that they'll have to sleep in the lobby overnight or go elsewhere, since people don't generally vacate rooms at 1am). This can cause fun things with rates: instead of completely blocking reservations or telling walkups that there are no cars available, the rental car company can just keep raising the rates. Eventually, the rates can reach ridiculous proportions: I've seen, in peak season, an economy car go out for $350 per day. The rationale is that for $350 per day, there is a car hidden in some corner somewhere (maybe one that's marked for an oil change that can be delayed) that can be coughed up, or for $350 per day, it's worth getting yelled at by a customer who does have a reservation but was forced to wait 20 minutes for a car because the last one on the lot was just given away to a $350 walkup. Or, the rental agency can offsell a $150 per day reservation to a competitor charging $200 per day and eat the difference because the $350 covers that difference. ($350 a day sounds cruel, but the alternative is to tell the walkup that there are no cars available, and--if everyone else in town is sold out, too--the customer will be left walking, taking taxis, or hitchhiking. The $350 is not a means to screw the customer; it's a way to signal that "we're sold out, but for a tempting offer, we might be willing to take some heat elsewhere to help you out.")
It's all made possible by the no-show factor. If people never changed their travel plans, it'd be easy to run a business in the travel industry--you could plan for exactly 100% occupancy and never have empty rooms or be oversold.
(Not all locations practice this type of yield management, though, and not all cities have tourist seasons with such strong demand...ANC is a very unique market, since intense demand is shoved into a three-month season, outside of which rates are in the single digits and all rental companies basically take a loss...)
