I'm not sure about an absolute beginning date, but the slogan was applied to PS-1 orders in the 1949-54 period.
It appeared on the right side of the car, with "The Overland Route" on the left side.
57400-57898 even, 1949, 40' auto car, 15' doors
106600-108598 even, 1949, 40' XM
6200-10198 even, 1951, 40' XM
1-625, 1953, 40' XM
all the above have riveted sides
656-1655, 1954, 40' XM
Not sayin' this is a comprehensive list. There was a another lot in 1955 that I'm not sure of offhand, and there were several small groups of cars built in the same time frame.
I should mention that a similar slogan, "Route of the "400" and the Streamliners" was used briefly pre-WW2.