ON RAIL-ROADS.

297

oilier: A A' is one line, which may be for thecarriages going in either direction, say fromA to A' and B B, the other line for the car-riages traversing the opposite direction, sayfrom B' to B; those two lines are supposedto extend the whole distance traversed, fromone end to the other. When the goods to beconveyed are to travel at the same rates ofspeed, perhaps few, if any, passings will berequired from one road to the other; but,when it is intended for the conveyance ofpassengers also, or for the transit of lightgoods at a swifter pace, then it will be neces-sary to have certain passings, so that the car-riages moving; faster can cross to the otherroad, and pass those moving slower; whenthey can again come upon their own road,and so proceed. Thus, suppose a train ofheavy goods travelling along the road Atowards A', and another train of lighter goodsor passengers coming in the same direction,the heavy train can then pass along the cross-ing a b into the road B B', when the lightertrain will pass, and the heavy one will againresume its former track, by proceeding alongthe crossing cd. In like manner, a light trainof carriages proceeding along the road B' Btowards B, encounters a heavy train travelling