Train Ferry
To ensure that the rail tracks on the train ferry or car float and the Linkspan align precisely it is necessary for the ship to have a ledge at its stern onto which the Linkspan is rested. To be certain that the rail tracks do not have a step at the junction of ship and Linkspan, this ledge or shelf must be of a depth the same as that of the end of the Linkspan. It is also fitted with a locating pin that ensures the Linkspan is in the exact athwart ships (sideways) position.
To protect the Linkspan from impact as the ship makes its final approach, stern fenders are positioned in front of it. These absorb the energy of the ferry’s impact, guide its stern and hold it from moving sideways when finally berthed. These guide fenders also prevent excessive loads being transferred to the locating pin.
As the trains roll onto or off the ship its freeboard and trim will change significantly. The Linkspan moving with the ship provides acceptable gradients which for railway traffic should not exceed 1:25 (4%). This relatively shallow gradient limited the areas where train ferries could operate. Where the tide is only 2 meters (6.56 ft) for example the Linkspan must have a length of at least 50 meters (164 ft). For any greater tide, the Linkspan must be very long; other problems also arise which can be very costly to solve.
Rail Linkspans are generally supported at their outer end by counterweights. This means that when the linkspan is lowered onto the ship’s ledge only a small proportion of its weight rests there. However half of the weight of the train on the Linkspan is transferred to the ledge. When it becomes necessary to make longer Linkspans to accommodate a greater tide range the train loads become proportionately higher until a critical reaction is reached. Before this point is reached, it is usual to create a second span with this inner span being adjusted at its outer end, where it is hinged to the outer span. Rail ferries must not only have the correct rail alignment, but their stern configuration and beam must be an exact fit for the berth it is to use.
Famous quotes containing the words train and/or ferry:
“The train rounds, bending to a scream,
Taking the final level for the dive
Under the river—”
—Hart Crane (1899–1932)
“This ferry was as busy as a beaver dam, and all the world seemed anxious to get across the Merrimack River at this particular point, waiting to get set over,—children with their two cents done up in paper, jail-birds broke lose and constable with warrant, travelers from distant lands to distant lands, men and women to whom the Merrimack River was a bar.”
—Henry David Thoreau (1817–1862)