A ferry slip is a specialized docking facility that receives a ferryboat or train ferry. A similar structure called a barge slip receives a barge or car float that is used to carry wheeled vehicles across a body of water. Often a ferry intended for motor vehicle transport will carry its own adjustable ramp - when elevated it acts as a wave guard and is lowered to a horizontal position at the terminus to meet a permanent road segment that extends under water. In other cases, the ramp is installed at the ferry slip and is called a linkspan or apron. Such a ramp is adjustable to accommodate varying water heights and ferry loadings and to move it out of the way during approach and exit. If railcars are carried by the ferry the apron will have tracks for them. In some parts of the world, the structures are also known as linkspans and transfer bridges. Similar structures are used to receive barges, particularly if the barge is for the carriage of railcars. San Francisco example In the example shown below, a tugboat was positioned on the left side of the barge (our right in the apron view), pulling it with a stout rope called a springline. Nearly identical structures were used around San Francisco Bay. Unlike the electric motor drive used here and elsewhere, the Point Richmond ferry slip used water tanks as a portion of the counterweight, with the amount of water (provided by gravity from a local creek) regulated to move the apron up or down by admitting or draining water from the tanks. Here, three tracks are provided for loading the barge. On the barge the three tracks are spread to allow clearance for the freight cars. The slip consists of pilings and guide boards to accurately position the barge relative to the apron. Once the barge is located properly, links are lowered from the apron to engage hooks on the barge, locking the linkspan and barge together. While the apron shown could bear the weight of a locomotive, it could not withstand the traction, so a string of flatcars was used to link the locomotive to a short string of railcars, which were then moved on or off of the barge. If a locomotive was to be moved (a rare event), it would be moved by another locomotive using the same method. The auxiliary track