Pending that revolution, tramway authorities are seeking to develop a cheaper means of electric traction than the tramway. At the present stage, urban tramways have spread through suburbs towards villages and small towns which are anxious for better transport facilities but have not sufficient population to justify a tramway extension. Inter-urban tramway systems—those connecting towns with a network of lines—are also adjacent to such minor centres of traffic. From time to time attempts have been made to meet the demand by means of petrol omnibuses, but they have rarely been successful—partly, no doubt, owing to the difficulty of working a limited petrol omnibus service economically at the extremities of an electric tramway system.
The latest solution of the problem is the 'trackless trolley' or, more correctly, the 'trolley omnibus.' In the 1911 session over a dozen tramway authorities applied for powers to use this device; and, if the financial results of the first attempts are successful, there will probably be a considerable growth in this type of electric traction.
The trolley omnibus is a hybrid between the trolley tramcar and the omnibus. It is akin to the first, because it derives its power from an overhead wire through a flexible trolley pole. It is akin to the second, because it does not run on rails but is fitted with solid rubber tyres and uses the surface of the road in the usual way.
Roughly speaking, its electrical equipment is similar to that of a tramcar. The trolley pole conveys the electric current to the controller, which admits it to motors geared on to the back axles. There are, however, one or two important differences. The absence of a rail which might act as a return conductor necessitates the provision of a second overhead wire and a second trolley-pole to connect with it. Thus the electrical circuit is from the power station, along the first overhead wire, down the first trolley-pole, through the controller and motors, up the second trolley-pole, and back by the second overhead wire to the power station. Owing to the vehicle being a steerable one, the trolley-poles have to be specially designed to give plenty of free play sideways. The vehicle itself is similar in appearance to a single-decked motor omnibus, and it runs on solid rubber tyres or spring wheels.
The first thing which strikes one about the trolley omnibus in comparison with the electric tramcar is the cheapness in first cost. All the expense of concrete foundations, heavy rails, and granite paving is avoided. On ordinary roads the overhead construction is much less costly, as a single line of poles supporting two wires is sufficient for the up and down services. Estimates show that the equipment of a mile of roadway on this system will cost only from one-fourth to one-third of the corresponding tramway system. Following on this economy there is the saving in the cost of maintenance and repairs—a serious item on the ordinary tramway. In actual working, the system has the advantage that the vehicles can steer past slow-going traffic, thus avoiding the delay caused on tramway systems through carts having to draw out, away from the track, when overtaken by cars. This steering or 'overtaking' power enables a trolley omnibus service to be maintained without obstruction on a narrow roadway which would be badly congested by tramcars running on a rigid track. When there is only one pair of wires, two trolley omnibuses may pass each other (whether going in the same or opposite directions) by the simple process of pulling down the trolley poles of one car and swinging them out of the way for a few seconds. On a single-line tramway it is necessary to provide loops at intervals for crossing purposes and also to arrange the service so that cars arrive at the loops simultaneously.
The other side of the picture is shown when we come to look into the costs of working.
No matter how good the road surface may be or how excellent the design of the wheel, the tractive effort required for a trolley omnibus must be relatively greater than that required for a tramcar. Nothing demands a lower tractive effort than a steel wheel running on a steel rail. Consequently the trolley omnibus takes more power per ton moved than the tramcar. When the road surface is wet or uneven, or muddy or loose, this difference is of course multiplied. Another addition to the working cost is produced by the tyres, which, if of rubber, may wear away at the rate of 1-1/2d. or 2d. per mile per vehicle. Owing to the uniform control of speed afforded by the electric system, there is less jerking at starting or stopping than is general with a petrol-driven omnibus; but in spite of that advantage, tyre wear on a trolley omnibus must remain an important item. Something must also be allowed for the effect of vibration upon the car body and electrical equipment—an effect which is of course much less pronounced when a vehicle runs on rails.
The balance between these advantages and disadvantages is not easy to strike, even on a general basis. And it varies so much under local conditions that tramway engineers debated a long time before they decided in certain cases to try the trolley omnibus in extending their traffic facilities. All they had to go upon was the experience gained on certain Continental routes, where trolley omnibuses have been running for several years. That experience encouraged the hope that trolley omnibuses might be a profitable means of developing traffic in conjunction with a tramway system, and along routes which would not provide sufficient business for a regular tramway.
The simultaneous adoption of the trolley omnibus on a number of tramway 'feeders' gave rise to an impression that tramway authorities had discovered the wheel-on-rail system to be less efficient than the tyre-on-road system. As a general proposition, nothing could be further from the truth. Tramway authorities have adopted the new system in certain cases where the possible traffic is comparatively small, not as a substitute for tramways, but as an alternative to self-propelled omnibuses. The carrying capacity of a trolley omnibus is about twenty, while that of a tramcar is frequently as high as seventy. The speed of a tramcar runs up to twenty miles an hour, while twelve miles an hour is as much as is comfortable (to say the least) with a vehicle running with solid tyres on an ordinary road.
Therefore, where large volumes of traffic have to be handled swiftly, the tramway will remain. But where a twenty-minute or half-hourly service of small vehicles is sufficient for the available passengers, a system which is much cheaper in first cost is clearly more suitable, even though it may not reach the standard of economy in working set by the large urban tramway. That is to say, the choice between the two systems depends entirely upon local circumstances.