KentuckyFC writes: Arbitrage is a way of making profit by exploiting price differences for the same asset. In capital markets, traders aggressively seek out and exploit these market "inefficiencies". Now one data scientist says it's possible to do the same with metro fares and has studied the fare-arbitrage potential of San Francisco's subway system, BART (Bay Area Rapid Transit). The idea is to swap tickets with another commuter during your journey to reduce the amount you both pay. BART has 44 stations which allows 946 different journeys and 446,985 unique pairs of trips. Of these, over 60,000 have arbitrage potential and commuters can save at least $1 on 4,666 of them. But there are good reasons why cities might want to maintain price differences for certain journeys--to encourage people to live in certain areas, for example. What's more, it’s possible to imagine a pair of commuters who each travel from one side of a city to the other at considerable cost. But by swapping tickets in the city centre, they could both pay for a short commute in each others’ suburbs. But is that fair to other commuters?