On every contract or agreement admitted to record relating to the sale of rolling stock or equipment, whether the title is reserved in the vendor or not, with a railroad corporation or other corporation or with a person, firm or company, the tax shall be 25 cents on every $100 or fraction thereof of the amount contracted for in such contract or agreement. When such contract or agreement is with a railroad corporation lying partly within the Commonwealth and partly without the Commonwealth, the tax shall be upon such proportion of the amount contracted for as the number of miles of the line of such railroad corporation in the Commonwealth bears to the whole number of miles of line of such railroad corporation.