The extended use of the bogie truck is an admission of its advantage over the fixed-wheel arrangement, both for distribution of weight and facility in passing round curves; but although it is now so largely adopted for engines and carriages on our home and continental railways, it is somewhat of an anomaly to find it so very rarely used for tenders. In the United States all the locomotive tenders--and many of them of very large size and weight--are carried on two four-wheel bogie trucks, and traverse the curves as easily as the engines. On this side of the Atlantic, the prevailing custom is to mount the tender on six rigid wheels; and as many of these tenders weigh as much as from 35 to 40 tons in working order, and have a rigid wheel-base of 15 feet, it will be seen at a glance that much unnecessary friction and wear and tear would be avoided by substituting two four-wheel bogie trucks for the fixed wheels. CHAPTER VII. 313 Signals--Interlocking--Block Telegraph and Electric Train Staff Instruments. Signals.--Railway tradition alleges that on one of the early lines opened for passenger traffic, the precautions for public safety were considered to have been fulfilled by providing a man on horseback to ride along the track between the rails in the front of the locomotive engine, to give warning to persons strolling on the line, and to check the advance of the train when necessary. A very short experience of this method of working proved that the full capabilities of the locomotive could not be obtained from a restricted speed of seven or eight miles an hour, and a more comprehensive system of signalling had to be devised. By fencing in on both sides of the line, the public were prevented from making a general highway or promenade along the railway, and the problem was reduced therefore to the signalling for the trains alone.