Railroads have played and continue to play an important role in national transportation systems, although the construction of new railroads on a large scale is something that belongs to the past. Nonetheless, railroads continue to be built such as those associated with high-speed networks. A vital part of a high-speed railroad, with trains travelling at speeds of up to 300 km h ^{? 1}, is the trackbed support. In other words, the dynamic behaviour of foundations and earthworks involves a detailed understanding of the soil structure interaction. This distinguishes a modern high-speed railway from other railways or highways. Obviously, the grades and curvature of railroads impose stricter limits than do those associated with highways. Furthermore, underground systems are being and will continue to be constructed beneath many large cities in order to convey large numbers of people from one place to another quickly and efficiently.

Topography and geology are as important in railroad construction as in highway construction (O Riordan, 2003). A good illustration of this has been provided by Baynes et al. (2005), who outlined an engineering geological and geomorphological approach to the construction of railways in the Pilbara, Western Australia. As noted, a very stable trackbed and earthworks are necessary for a high-speed railroad. Accordingly, drainage is an important aspect of trackbed design, so that surface water is removed efficiently and groundwater level is maintained below the subgrade. Another important factor is trackbed stiffness. Consequently, where sudden changes of stiffness are likely to occur, notably between embankments and...