2 cents. It isn't from 1870, that date refers to the date the train was in operation. In mint condition you can use it for 2 cents of postage. These were issued in coils and there are certain plate numbers that might bring a dollar or two.
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First of all, this stamp is not from the 1870's, it is indicating that the locomotive pictured on the stamp was from the 1870's. The stamp itself has a minimal value and can be purchased for 15 cents new or used. You probably won't find anyone willing to pay you for it unless it can be used for postage and then it would be face value.
That stamp is Scott Number US 1897. It was issued in 1983. It has a value of 20 cents used and mint.
The stamp was issued in 1982 and is Scott Number US 1897A. It has a minimal catalog value.
That would be US Scott Number 2226 issued in 1987 or 1897A issued in 1982. They both have a minimal value of 20 cents mint and 20 cents used.