Alignment considerations weigh more heavily on railway design versus highway design
for several reasons. First, unlike most other transportation modes, the operator of a
train has no control over horizontal movements (i.e. steering). The guidance
mechanism for railway vehicles is defined almost exclusively by track location and thus
the track alignment. The operator only has direct control over longitudinal aspects of
train movement over an alignment defined by the track, such as speed and
forward/reverse direction. Secondly, the relative power available for locomotion
relative to the mass to be moved is significantly less than for other forms of
transportation, such as air or highway vehicles. (See Table 6-1) Finally, the physical
dimension of the vehicular unit (the train) is extremely long and thin, sometimes
approaching two miles in length. This compares, for example, with a barge tow, which
may encompass 2-3 full trains, but may only be 1200 feet in length.