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. [Illustration: Fig. 7. The 'auto-trolley' system of electric traction applied to the haulage of goods in a German quarry. (From _Electrical Industries_.)] As an emphasis upon this statement, it is significant that many tramway engineers regard the trolley omnibus merely as the forerunner of a tramway. For this reason they favour the adoption of the particular trolley omnibus system where the overhead equipment is adaptable with trifling changes to tramway purposes. They argue that, in the case of a village of a few thousand inhabitants, situated a mile or so beyond the terminus of a tramway route, a trolley omnibus service will not only be sufficient for the existing traffic, but will show whether the traffic is likely to increase (through the stimulation of building enterprise) up to the point where it would make the laying of rails worth while. When that point is reached, the rails will be laid and the trolley omnibus vehicles put on some other route which is at one and the same time a tramway 'feeder' and a tramway 'feeler.'