What is a Interchangeable Mileage Credential?

[wrobwrob]

Can somebody explain the Interchangeable Mileage Credential issued by the Southwestern Passenger Mileage Bureau?

I am looking at a tan, "tri-fold" leather wallet, 3-1/2" x 6-1/2" when closed. When open the left is a compartment for holding trip receipts. Pasted in the center is a Mileage Credential contract, No. SB 38553. It is stamped GH&SA Ry. Co., Dec. 27, 1909, El Paso City Office. Pasted on the right is a list of rail lines where the Mileage Credential can be used.

From reading the contract my guess is that this was something used by traveling salesmen or other frequent travelers, because it allows the purchaser to submit trip receipts and get a refund of the excess over 2-1/2 cents per mile. It was good for one year. The area covered is basically New Mexico and Colorado east to the Mississippi.