A conductor, guard (British English), or travelling ticket examiner (Indian English), is a member of a train crew responsible for operational and safety duties. The role is common worldwide under various job titles, although on many railroads, the role has been discontinued. The title conductor is most common in North America, Australia and most other English-speaking countries outside of Great Britain and India, where the title is guard or travelling ticket examiner, respectively.

The responsibilities of the role typically include:

Some rapid-transit systems employ conductors to make announcements and open and close doors, duties otherwise performed by train drivers. The conductors often stay in the center of the train, where they have the best view of the platform. However, most rapid-transit systems are driver-only-operated.

In North America, the conductor manages a freight, passenger, or other type of train, and directly supervises the train crew, which can include a brakeman, flagman, ticket collector, assistant conductor, and on-board service personnel, and is responsible for the movement of the train. The engineer (driver) takes direction from the conductor. The portman (station master), and additional engine crew members (fireman, pilot engineer) share responsibility for safe and efficient train operation and adherence to railway rules and procedures. On some railroads, union contracts specify that a conductor must progress to engineer.

Other duties carried out by the conductor include:

Passenger trains may employ one or more brakemen/assistant conductors who assist the conductor and engineer in the safe and prompt movement of the train, to share the workload, and accept delegated responsibility. If a train crew's route, or tour of duty, exceeds a single shift, or conflicts with a legal or contractual limit on the number of work hours, more than one crew may be assigned, each with his own brakeman/assistant conductor. On-board service crew members on passenger trains normally remain on duty for the entire run, including assigned meal and sleep breaks.

Since nearly the beginning of railroading in North America, the conductor on freight trains rode aboard a caboose, along with the rear flagman and the rear brakeman, and performed duties from there. Advances in technology and pressure to reduce operating costs made cabooses redundant, and in most cases, they have been eliminated. This relocated the conductor from the rear of the train to the locomotive (or locomotives) at the head of the train. In most cases, these same conditions gradually eliminated members of the train crew, such as the head brakemen, flagmen, and others.

Most freight trains on most railroads today have a crew of two, one engineer and one conductor. Railroad companies continue to press for reduced operating and labor costs, and this threatens to eliminate second men. Railroads rationalize that since the engineer is already qualified as a conductor, he can easily assume the duties of a conductor. In fact, on most railroads, engineers begin as brakemen, then become conductors, and finally engineers. Some railroads already implement such a strategy, notably the Montana Rail Link, and operate with an engineer and a conductor. However, most railroads are contractually obligated to employ a brakeman/assistant conductor in addition to the engineer, via crew-consist agreements negotiated with the major rail unions, primarily the United Transportation Union. Therefore, eliminating the conductor's position would require that the railroads and unions negotiate a new agreement. If the railroads were successful, conductors already trained and certified as engineers would theoretically be able to work as engineers. Those who have not yet progressed to engineer would have to be trained as engineers as positions became available. Others would have to accept other positions or possibly lose their jobs. The primary union for engineers, the Brotherhood of Locomotive Engineers, does not support this movement, claiming that requiring its members to operate trains alone would be unsafe.