The Collaborative International Dictionary of English v.0.48:

Railroad \Rail"road`\ (r[=a]l"r[=o]d`), Railway \Rail"way`\
   (r[=a]l"w[=a]`), n.
   1. A road or way consisting of one or more parallel series of
      iron or steel rails, patterned and adjusted to be tracks
      for the wheels of vehicles, and suitably supported on a
      bed or substructure.
      [1913 Webster]

   Note: The modern railroad is a development and adaptation of
         the older tramway.
         [1913 Webster]

   2. The road, track, etc., with all the lands, buildings,
      rolling stock, franchises, etc., pertaining to them and
      constituting one property; as, a certain railroad has been
      put into the hands of a receiver.
      [1913 Webster]

   Note: Railway is the commoner word in England; railroad the
         commoner word in the United States.
         [1913 Webster]

   Note: In the following and similar phrases railroad and
         railway are used interchangeably:
         [1913 Webster]

   Atmospheric railway, Elevated railway, etc. See under
      Atmospheric, Elevated, etc.

   Cable railway. See Cable road, under Cable.

   Ferry railway, a submerged track on which an elevated
      platform runs, for carrying a train of cars across a water
      course.

   Gravity railway, a railway, in a hilly country, on which
      the cars run by gravity down gentle slopes for long
      distances after having been hauled up steep inclines to an
      elevated point by stationary engines.

   Railway brake, a brake used in stopping railway cars or
      locomotives.

   Railway car, a large, heavy vehicle with flanged wheels
      fitted for running on a railway. [U.S.]

   Railway carriage, a railway passenger car. [Eng.]

   Railway scale, a platform scale bearing a track which forms
      part of the line of a railway, for weighing loaded cars.


   Railway slide. See Transfer table, under Transfer.

   Railway spine (Med.), an abnormal condition due to severe
      concussion of the spinal cord, such as occurs in railroad
      accidents. It is characterized by ataxia and other
      disturbances of muscular function, sensory disorders, pain
      in the back, impairment of general health, and cerebral
      disturbance, -- the symptoms often not developing till
      some months after the injury.

   Underground railroad Underground railway.
      (a) A railroad or railway running through a tunnel, as
          beneath the streets of a city.
      (b) Formerly, a system of cooperation among certain active
          antislavery people in the United States prior to 1866,
          by which fugitive slaves were secretly helped to reach
          Canada.

   Note: [In the latter sense railroad, and not railway, was
         usually used.] "Their house was a principal entrep[^o]t
         of the underground railroad." --W. D. Howells.
         [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Underground \Un"der*ground`\, a.
   1. Being below the surface of the ground; as, an underground
      story or apartment.
      [1913 Webster]

   2. Done or occurring out of sight; secret. [Colloq.]
      [1913 Webster]

   Underground railroad or Underground railway. See under
      Railroad.
      [1913 Webster]

WordNet (r) 3.0 (2006):

Underground Railway
    n 1: secret aid to escaping slaves that was provided by
         abolitionists in the years before the American Civil War
         [syn: Underground Railroad, Underground Railway]