The general usage of the phrase electric train, is referred to a whole unit consisting of large number of coupled carriages (passenger or freight) hauled by an electric locomotive instead of the earlier steam locomotive, or a unit with smaller number of coupled carriages which have independent electric drive motors underneath the chassis (undercarriage), controlled by the driver at one end of the train, from a small cabin. The difference is only in the type of hauling or drive mechanism used. The latter form is common in public transport, where a drive car can be coupled with one or two other cars to form a group that can be easily joined to another group at peak travel times.
The long journey train has many carriages, which are hauled by a locomotive at one end, and some times two, one at each end, if additional power is required to haul additional load or up an incline.
The locomotive for the above purpose will be either diesel-electric or only electric. In diesel electric locomotive the diesel engine generates its own electric power and the power is utilised to drive the electric motors on the locomotive only.
The electric locomotive, in contrast, draws electric power from an outside source, say by an overhead conductor through a pantograph or similar collector device, which can make contact or retract. The power is utilised to drive the electric motors on the locomotive only.
For diesel electric locomotive and electric locomotive, the carriages may be same as the steam locomotive. However for the electric train, the carriages are designed with drive motors almost on each carriage mounted underneath each carriage chassis (under carriage).
Generally for long haul trains as met with in India, the steam locomotives have been replaced by diesel electric locomotives. This is also being replaced gradually by electric locomotives in view of various comparative advantages.