railways which have to be constructed in this manner, the trains must necessarily be lighter, and the average speed must be considerably less than where the conditions are more favourable. Such lines are, therefore, in proportion to the traffic carried over them, more expensive to work and maintain, but as the weight of the train, the class of locomotive used, and all the other arrangements for conducting the business are specially adapted to the circumstances, such a line as this probably involves less trouble and anxiety in working than a railway which is generally level, but which has one or more long and steep gradients in its course. In the latter case, the steep gradient is the exception, and not the rule; all the conditions are adapted to the maintenance of the high rate of speed expected with heavy trains on fair gradients, and yet the engines must have sufficient adhesion and steaming power to surmount the difficulties of the exceptional gradient. In some cases the plan is adopted of attaching an auxiliary engine (called a "bank engine") to the rear of the train at the foot of the incline, but this involves an additional stoppage and the loss of valuable time, and special precautions are required to enable the operation to be performed with certainty and safety.
The extent to which the average speed of trains is affected by the mode of construction of a railway is strikingly illustrated by the following table, which gives particulars of the curves and gradients, and the speed of the trains, upon the main line of the London and North-Western Railway from London to Carlisle, upon the Buxton Branch, the Central Wales line, and the Merthyr, Tredegar, and Abergavenny Branch.
