As I mentioned at the beginning, tank cars are very heavily regulated. If the AAR says it's TC-128, one-half inch, I cannot change it. I cannot go with less. I can go with more; but I cannot go with less. If they say a valve should be within this pressure, it has to be that. I cannot change it. What we do when we change it is through our detail design. We design railcars based on a 50-year life. It doesn't matter what type. Based on AAR, when you design a freight railcar, it goes for 50 years. That means if they want it to go beyond 50 years, they have to take it out of service, visit it, check it, recertify it for extended life. That's one.
The second one is we design it for fatigue life. To design any freight cars, you have to look at different aspects of design. There's static design, dynamic design, fatigue design, buckling, and model. One of the key design factors is fatigue life. These railcars are under a lot of vibration. It is not like our cars that have very soft suspension and you don't feel the vibration. It's steel on steel so it gets lots of vibration. So on fatigue life for a railcar, again as specified by AAR, it says for interchange cars it has to be a 1-million mile minimum. For cars, a unit, a train service, like intermodal cars, it has to be a minimum of 3 million miles. What NSC is doing with our cars is going beyond that. Our design is for 5 million miles. That is differentiating us from others, and some customers are asking for that, and they pay for that. It doesn't come for free. This is like options on your car; if you want that, it's different.
Generally speaking, on the details, we protect our IP and we make small changes. But on tank cars, it's not big items. The big items are fixed: that's the minimum, you have to meet it.