Final answer:
A decrease in the price of rail travel leads to a leftward shift in the demand curve for airline travel, signaling that consumers demand less air travel at every price level. This shift is due to consumers potentially seeing rail travel as a more attractive substitute for air travel when it gets cheaper.
Step-by-step explanation:
When the price of traveling by rail decreases, it is likely to affect the demand for airline travel. Consumers may see rail travel as a substitute for air travel, so when rail becomes cheaper, they may prefer to use the train instead of flying. This change leads to a decrease in demand for airline travel, causing the demand curve to shift to the left. This shift means that at every price point, the quantity demanded for airline travel will decrease. We do not move along the supply curve; instead, the entire demand curve for air travel shifts.
It is important to note that this analysis assumes ceteris paribus, meaning all other factors remain constant. If other factors are changing simultaneously (like a change in the cost of jet fuel), the total effect on the market for airline travel could be different. The specific impact on the equilibrium price and quantity would depend on the relative magnitude of the shifts in the supply and demand curves for air travel.