These expedients are necessary only on railways where traffic is very heavy. On most railways (on all where the total delay does not exceed five hours per day) it is cheaper to load on the main track until the first cut has been made. This necessarily involves the delay due to running to and returning from the nearest side track to get out of the way of every main line train, until the pit track is long enough to contain the construction train. This, however, seldom requires more than two weeks, generally only one; the excavation of all of the first cut does not often occupy more than a month, and is only a very short time compared with the whole length of time that the steam shovel is usually in operation on all but very small jobs.
Fig. 26.
After a side track has been laid in the first cut made by one of the methods described above, the steam shovel begins work at A, [Fig. 26], loading cars standing on the side track, and some of them extending out on the main track. At first not more than ten cars should be coupled to the engine, so that the train can quickly run into the side track on the approach of a main line train, and not delay its passage. After the steam shovel has advanced a train-length, the full number of empty cars can be coupled to the engine, as they will all be on the side track while being loaded.
Fig. 27.
Where the embankment has been previously widened by the excavated material from the cut, [Fig. 27], a sufficient length to permit laying a side track long enough to hold the construction train, the full number of cars can be used at once, a great advantage in keeping the steam shovel at work without interruption by passing trains, which is unavoidable when some of the cars extend out on the main track.
After the machine has reached the other end of the cut it is either withdrawn for other work, or placed on the other side of the main track for widening the cut on that side. The steam shovel begins at A, [Fig. 28], loading cars standing on the main track; the main line traffic being carried over a temporary main track built in the excavation previously made by the steam shovel on the other side of the main track. Only a few cars at a time can be used for loading at first, unless the temporary main track has been extended toward B a sufficient length to clear the usual string of about 20 cars when the first car is being loaded.