Weight Considerations

Given that the density of aluminum is approximately one-third that of steel, the notion that the weight of a structure constructed of aluminum will be substantially less that of a comparable steel structure is widely contended. Clearly, this is not the case since more aluminum is required to construct a structure of equivalent strength to that of steel and it is may not be practical (based on design constraints) to build a rail car which is 100% aluminum (or 100% steel). Items such as windows and seats for example contribute substantially to the total car weight. Trucks and truck components (wheels, axles) are always constructed from steel. Lighter weight materials can have the most significant impact on the total car weight when used for the car shell and structural members as shown in Figure 2, below.

Figure 2. Relative railcar weight based on construction material [[12]].

Car weight is an important consideration in train operations, especially when propulsion systems are challenged as may be the case for the envisioned PRIIA 125 mph diesel locomotive. In a train, the total weight that must be moved divided by the number of passengers yields a "pounds per passenger" factor which is directly proportional to the amount (and cost) of the energy required to move that passenger. Minimizing the “pounds per passenger” factor can translate into substantial savings for transportation authorities. In almost all cases, the "pound per passenger" factor is lower in trains comprised of aluminum cars, since aluminum cars are generally somewhat lighter.

Figure 3 presents a survey of Bombardier-made products for the subway market [13]. The survey was performed to compare certain characteristics of aluminum and stainless steel cars for this service application. Although dissimilar to the subject PRIIA bi- and single-levels cars this information is of particular interest here.

The upper portion of Figure 3 describes the weight per inch of car length and suggests that as cars increase in length the weight reduction benefit achievable with an aluminum design diminishes. The PRIIA cars for which specifications have been developed are to be 85 feet (~1000 inches) in length and the data in Figure 3 suggest (if it is assumed that the trends remain when scaled for longer, heavier vehicles[1]) rather modest weight savings. This is corroborated by the data presented in Figure 2.

Figure 3. Comparison of relative weight and buff strength for aluminum and stainless steel SUBWAY vehicles [[13]].

The lower portion of Figure 3 compares the compressive (buff) strength of stainless and aluminum subway cars (in terms of the car weight per inch of length) and indicates that (albeit with somewhat sparse data) equivalent buff strength can be achieved with a 25% lighter aluminum car with the same weight/unit length ratio.