When a strained film is grown on a vicinal substrate, the steps advance like a train when the deposited atoms have sufficient mobility to reach the step edges. However, as the steps advance, the strain-induced force monopoles associated with the steps cause the steps to attract to each other (J. Tersoff, PRL 74, 4962, (1995)), resulting in a thermodynamic instability of the steps in the form of step bunching (J. Tersoff, et al., PRL 75, 2730 (1995)).