Section 143. Every railroad corporation shall, within four months after receiving a written request therefor, furnish to any county, city or town in the commonwealth, or to the department of highways, as the case may be, a sufficient number of such warning signs to enable such county, city or town, or the said department, from time to time to comply with the preceding section. The said signs shall be furnished, as aforesaid, without charge, unless they are to be used for replacement purposes, in which case the railroad corporation may require the payment of the net cost thereof.