No electric car shall be operated for the purpose of carrying passengers which has a flat wheel, broken window or windowpanes, broken door or door panes or a leaky roof, side or other part thereof. All corporations operating electric railways shall have and keep in constant operation a sufficient number of convenient, comfortable and clean cars to transport the usual number of persons desiring transportation and shall, in addition thereto, keep in reserve a sufficient number of cars, power, equipment, ways, means and facilities to supply transportation in case of emergency. But the penalties provided for the enforcement of this section shall not apply when it can be shown that the damaged equipment was used only on the day such damage occurred.