230 POWER DISTRIBUTION FOR ELECTRIC RAILROADS. 
each, running at-a speed of twenty-five to thirty-five miles 
per hour, including stops. 
In the present state of the art, this is not a serious 
matter. The only material difficulties that have been met 
in practice are those connected with the delivery of the 
necessary current to the moving car, and these are not 
now of much moment. 
The actual amount of power used for such service is 
easy to compute. Taking for a unit a train composed of 
one long motor car and one trail car, capable together of 
accommodating nearly two hundred people, we can derive 
the necessary power. The weight of the two cars complete 
would be about fifty tons of which about thirty tons 
would belong to the motor car and twenty to the trailer. 
Allowing for ten tons live load the total weight of the 
loaded train is sixty tons. 
The tractive power per ton may be taken direct from 
railway practice since the roadbed and rails are, or always 
should be, the same ordinarily used in steam railroading. 
For such track and speed the tractive coefficient should 
never be more than 12 lbs. to 15 lbs. per ton. Taking the 
latter figure as covering all ordinary contingencies of curves, 
etc., the horizontal effort becomes goo 1bs.; to this must be 
added the air resistance, and whatever resistance may be 
due to grades. At thirty miles per hour the air resistance 
is between 3 1bs. and 4 1bs. per square foot of surface normal 
to the direction of motion. 
Allowing 200 1bs. for this factor of the resistance we 
have a horizontal tractive effort of r1oo lbs. and there 
would be required at thirty miles per hour the expenditure 
of eighty-eight mechanical horse power. 
Maintaining this speed of thirty miles per hour on 
grades, the additional horse power required would be 
ninety-six for each per cent of grade, or dropping the 
speed to twenty miles per hour on the grades, sixty-four 
horse power for each per cent of grade. 
Allowing about eighty per cent net efficiency from 
the motor terminals to the wheels it appears that the elec- 
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