The cars should also be provided with sheet iron aprons, Figs. 82 and 83, extending from the end of one car onto the floor of the next, to prevent the material from falling on the track between the cars as the plow is drawn over them, and delaying the departure of the train until it can be shoveled out. These aprons are made either in two pieces, Fig. 82, or in one piece only, Fig. 83. The former are more easily handled, and permit access to the coupling of the cars without lifting the apron. Very little material drops on the track when the aprons and the center plow are used. The single apron is used mostly in connection with the side plow.
The number of cars and engines required for each steam shovel to keep it in nearly constant operation depends upon the nature of the material excavated, the length of haul, and the density of other traffic upon the main line. This number must be determined by accompanying circumstances in each case; ordinarily, however, it averages about as given in Table III.:
In the steam (--------------On the road up to-------------) (shovel pit.) 10 miles. 25 miles. 50 miles. 75 miles. Loco. Cars. Loco. Cars. Loco. Cars. Loco. Cars. Loco. Cars. Loose gravel 1 30 1 30 2 60 3 90 4 120 Dry clay 1 22 1 22 2 40 - - - - Damp stiff clay 1 18 1 18 2 36 - - - - Hardpan, cemented gravel, etc., loosened by explosives 1 16 1 16 2 32 - - - -
The length of haul usually ranges from 2 to 15 miles; it seldom exceeds 25 miles for any material except gravel ballast, where hauls of 75 miles are frequent, and sometimes reach 200 miles.
On hauls exceeding 25 miles the full number of cars and engines required can seldom be obtained, and the output of the steam shovel is correspondingly decreased. The delay in returning empty cars due to detentions from other trains is the great trouble most keenly felt in steam shovel work on railways in operation. The so-called "mud train" is generally considered an outcast, and is usually the last train to receive the dispatcher's attention for an order to the road. These delays are daily occurrences, and it is quite an exceptional case when the machine is amply supplied with empty cars. The record of most steam shovels on such work is therefore a rather poor one, when the machine really made a good showing for the crippled condition of its car service. Some of these delays can be avoided or shortened by stationing a telegraph operator at the outgoing end of the pit, and on all but very small pieces of work his wages will be many times balanced by the time gained in keeping the whole plant moving, by obtaining train orders quicker, and remaining constantly informed of the whereabouts of the construction and other trains, and regulating the work in the pit accordingly.