passenger mile

from The Century Dictionary.

• noun A unit of railroad accounts which represents the transportation of one passenger for the distance of one mile.

from the GNU version of the Collaborative International Dictionary of English.

• (Railroads) A unit of measurement of the passenger transportation performed by a railroad during a given period, usually a year, the total of which consists of the sum of the miles traversed by all the passengers on the road in the period in question.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A basic unit used by transport companies for calculating profit levels etc. It is calculated by dividing total distance travelled in a given period by the number of passengers.