Turnstile in the context of "One-way traffic"

A train ticket is a transit pass ticket issued by a railway operator that enables the bearer to travel on the operator's network or a partner's network. Tickets can authorize the bearer to travel a set itinerary at a specific time (common for long-distance railroads), a set itinerary at any time (common for commuter railroads), a set itinerary at multiple times, or an arbitrary itinerary at specific times. The last two categories are often called passes: the former is often sold as a discounted block of trips for commuters; the latter is often sold to vacationers, such as European Eurail passes.

In some countries, like Italy, and some local railways in Germany, conductors are not used. Instead passengers are expected to validate tickets in a special stamping machine before entering the train. A system of coupons that are validated with a special machine exists on the Mumbai Suburban Railway where combinations of coupons of denominations are used to get the corresponding ticket value. There may or may not be a conductor later on double-checking that correct tickets are actually held. Yet further systems are possible, for example in Japan, the London Underground and in local traffic in Stockholm, the platforms are blocked by turnstiles, forcing the acquisition of a ticket before entering the platform.

In economics, the free-rider problem is a type of market failure that occurs when those who benefit from resources, public goods and common pool resources do not pay for them or under-pay. Free riders may overuse common pool resources by not paying for them, neither directly through fees or tolls, nor indirectly through taxes. Consequently, the common pool resource may be under-produced, overused, or degraded. Additionally, despite evidence that people tend to be cooperative by nature (a prosocial behaviour), the presence of free-riders has been shown to cause cooperation to deteriorate, perpetuating the free-rider problem.

In social science, the free-rider problem is the question of how to limit free riding and its negative effects in these situations, such as the free-rider problem of when property rights are not clearly defined and imposed. The free-rider problem is common with public goods which are non-excludable and non-rivalrous. The non-excludability and non-rivalry of public goods results in there being little incentive for consumers to contribute to a collective resource as they enjoy its benefits.