Figure 60,000 miles a year under the freights (thats running the CZ route about 120 times a year), then about 100,000 miles a year on Amtrak until 1982, then a function of an increase of about 3000 miles a year thereafter.
So... For a 1948 car...
1948: 60,000 1949: 120,000
1950: 180,000 1951: 240,000 1952: 300,000 1953: 360,000 1954: 420,000
1955: 480,000 1956: 540,000 1957: 600,000 1958: 660,000 1959: 720,000
1960: 780,000 1961: 840,000 1962: 900,000 1963: 960,000 1964: 1,020,000
1965: 1,180,000 1966: 1,240,000 1967: 1,300,000 1968: 1,360,000 1969: 1,420,000
1970: 1,480,000 1971: 1,580,000 1972: 1,680,000 1973: 1,780,000 1974: 1,880,000
1975: 1,980,000 1976: 2,080,000 1977: 2,180,000 1978: 2,280,000 1979: 2,380,000
1980: 2,480,000 1981: 2,580,000 1982: 2,683,000 1983: 2,789,000 1984: 2,898,000
1985: 3,010,000 1986: 3,125,000 1987: 3,243,000 1988: 3,364,000 1989: 3,488,000
1990: 3,615,000 1991: 3,745,000 1992: 3,878,000 1993: 4,014,000 1994: 4,153,000
1995: 4,295,000 1996: 4,440,000 1997: 4,588,000 1998: 4,739,000 1999: 4,893,000
2000: 5,050,000 2001: 5,210,000 2002: 5,373,000 2003: 5,539,000 2004: 5,708,000
2005: 5,880,000 2006: 6,055,000 2007: 6,233,000 2008: 6,414,000 2009: 6,598,000
Today: 6,661,333